You would have had to have a dictatorship implemented to hold out, which there was no appetite from anyone to do
But might there not have been an appetite to submit to some sufficiently firm and "visionary" dictator, if this were the only way to make a really profitable system politically viable?
I think you overestimate the British abolition as an expression of British virtue anyway, and are ignoring the geographic-economics of capitalist slavery. Basically, slavery is favored in a capitalist system when there are opportunities for intensive production in locations where the surrounds would offer a lot of refuge and alternative means of survival to a labor force; with some ASB thing (or a realistic world society that has moved on to abolish slavery) compelling free labor only, wages would be sky high and yet intensive overworking of the labor force would have limits--severe ones from a capitalist point of view, in fact. Importing forced labor and chaining it down is rational then.
But when such a site, be it for mining or plantation work, fills up, when political control of the countryside is strong and escapees have no where to run, and the land is all bought up and refugees have no place to subsist independent of the labor market--then rationality tips toward free labor.
By the 1830s, the sugar islands had had something like a century to populate and spread such control over, while in America fresh frontier lands for slave plantations still beckoned. This I think accounts for the thirty year gap--even so, under the British system other forms of constrained servitude, even those as close to plain old slavery as blackbirding, not to mention "coolie" contract-labor, were quite prominent. Also, as I understand it it was pretty common in European colonies including British ones to do stuff like impose head taxes which could only be paid in the currency of the realm, which, in colonized territories, was not a form of money that could be gotten by traditional economic trading but only by going to work for some colonial enterprise; this in effect forced a labor draft.
It may not be reasonable to denounce the British system as the very abyss of depravity, though the OTL scale of it, casting the net onto every continent on the globe, explains the apparent hyperbole of Irish people muttering about the Enemy of Mankind as literal and sober observation. I think your resistance to the idea that it was somewhat dystopian OTL relates to general resistance to the idea liberal capitalism can be dystopian, but I think any reasonable consideration of facts shows how it is in fact generally so, for all more or less liberal capitalist regimes.
The thread OP question is can it be both more extensive--it doesn't raise the question of durability and we can go any way we like on that--and also more dystopian. And more dystopian has several dimensions.
There is a conflict between being more illiberal and more successful, in that certainly much British success related to a deepening culture of liberalism. On one hand this meant more legitimacy for loyalist democracy, for such movements as Chartism to be compromised with with gradual extension of franchise and gradual legitimizing of claims of common working people, which had some salience to the rising claims of colonized peoples to consideration as well, versus the hard-edged Social Darwinism linking the British establishment to success or failure in market terms. OTL British dominance of the global capitalist system peaked in part due to the ornery resistance of British firms to being cartelized, along German or American models, into largely centralized super-enterprises that ruthlessly rationalized the general industrial system. British industry rose to great heights collectively then reached a plateau as German and American industry caught up and surpassed it. In America a facade of decentralizing ideology, known as "trust busting," put a fig leaf of deniability on the ongoing centralization of control into the hands of great corporations; in Germany they were more forthright and the central cartels were acknowledged and assumed as basically necessary and progressive in terms of efficient innovation in modern terms.
So, had Britain remained more forthrightly class-stratified, with the franchise for Parliament ruthlessly reserved for the propertied, perhaps that would have undermined the growth of industry in the 19th century phase when enterprises relatively small compared to the kind of firms that would dominate by say 1900 were favored, and perhaps Continental powers and the USA would pull even and surpass even earlier. The topic of demographics has come up; could the people of one damp island spread out enough, with enough loyalty, to ruthlessly dominate the globe even more than OTL? In South America British success was by indirect means; everywhere, in the formal dominions and protectorates as well as in the informally dominated places British influence radiated via local elites raised up by association with British interests and more or less following British orders, largely through private channels. If the British system were more formally centralized, might the ability of British influence via such indirect channels have been stunted?
Would a more dystopic empire be one where all British subjects, including those born in Britain, be under a harsher plutocratic rule, or might we have as much or more democracy in Britain linked to a racist white-supremacist ideology mobilizing even low class Britons to hold themselves a master race organized to firmly rule "lesser breeds without the law" as Kipling so bluntly put it in "White Man's Burden?"
I don't think it is strictly necessary Britain hang on to the OTL USA, or even frustrate independent US ambitions in continental "Manifest Destiny;" we can go either way with this. Nor is it necessary that the various "white dominions," including perhaps a successfully retained British North America, be run in a more conciliatory fashion, to perhaps still have a reservoir of loyalist colonials who can be relied on as imperial troops to dominate others.
There are lots of ways to go with this.
My personal taste is biased toward being seduced by the prospect of a less dystopian but more extensive and durable Empire, and that is down to frankly some bias toward a more romantic view of the Anglosphere generally and a notion that unity is better than division--provided the united entity is reasonably just. I do think, soberly, that empire and justice are in conflict fundamentally, that no great power exists without a lot of brutality and ruthlessness. Britain could not rule a world empire on progressively liberal lines, without turning into an Indian Empire, which would not happen; if Britain could not maintain a Raj on the basis of white supremacy then sooner or later they'd be out of India, and without India the Empire remaining tends to fall apart. Perhaps an ATL where India does not come under unified British rule via the EIC but remains partitioned between rival European powers and perhaps eventually some Indian principality gains the ability to toss the lot of them out, but Britain does keep a grip on North America instead, could compensate for no-India or just part of India held marginally with American resources. But again fully exploiting American resources will tend strongly to turn the Empire into an American Empire, even if the British grip on North America is on rather illiberal lines, which I think likely to evolve either if the Revolution is preempted or if the Patriots (Rebels in this TL of course, and much deplored) are overcome. The key to ongoing British control of North America would be via firmly establishing an American gentry, and might go hand in hand with general illiberalism. For an illiberal Britain to dominate the 19th century it would be necessary to somehow coordinate the rise of industrialism with a less laissez-faire political ideology, to hold that key and important minorities ought to rule while still keeping their status tied to differential success or failure industrially, and such a regime will be beset by ongoing grassroots radicalism, which need not triumph to persist.
In such a setup, I suppose colonial forms of racism would be imported into the home islands more so than OTL (where it was quite bad enough, not American in extremism to be sure, but in the ATL it might be and perhaps worse).
There is a lot of ATL potential here, I wouldn't delve into it too deeply because it is ugly and painful, not because I doubt it might be a plausible possibility.