I'm not a China expert by any means, so don't take this the wrong way.
Of course not; neither am I (and I'm still trying to do some research for which China would be part of a few TL projects I'm considering).
But it seems like a much easier way is to have the UK have relations with ROC. What are the factors for and against?
See, here's how I would view it. Up until 1950, it would certainly be possible for the UK to have some form of relations with the ROC
had it retained a substantial portion of territory in China itself. Even that, however, is not all that certain because:
a) Jiang was very much adamant in wanting to end all the foreign concessions resulting from the unequal treaties and all that. While the war with Japan was a major factor, other countries before and after the war with Japan also managed to end their concessions and hand their territory over. The expectation here would be that Britain would do the same WRT Hong Kong after the war was over.
b) OTOH, Whitehall was pretty much sceptical (British spelling intentional here) of Jiang's regime, and was absolutely baffled when FDR started promoting China as one of the major Allies of the War. Even then, everyone knew that if the Civil War was going to re-ignite, the Communists were going to win regardless, so it was a pointless waste of time to prop up a corrupt regime like Jiang's. So the UK held a wait and see attitude when it came to the Civil War (at least as far as HK is concerned, since it was highly dependent on imports of food, water, and electricity from the Mainland), and then switched to recognizing the PRC almost immediately after Mao declared it from Tien'anmen Square.
So, from that viewpoint, the UK was never really going to recognize Jiang's regime on Taiwan; there could be a presence there, if need be (which the UK ultimately did on the model of
the US's AIT), but it would not be official recognition - all the more so since, up until the Korean War, everyone was expecting Taiwan would be one of the last dominoes to fall to the PRC's authority. So, not only would holding substantial territory in China itself would not just be enough (and, more importantly, having more areas in China under the direct jurisdiction of the central government instead of in the hands of warlords), but the nature of the ROC itself would have to dramatically change.
That is why I presented other solutions to that conundrum that could make the photoshop seem plausible, since no matter which Chinese government takes control, they're all going to get HK anyway, sooner or later.
The ROC on Taiwan managed to drastically change because Jiang and his government had such a thorough loss of face from being on the losing side of the Civil War that they needed to rebuild his credibility. You would need something on that type of scale much sooner to have even a modicum of change of scepticism in Whitehall. My personal favorite would be having the China Democratic League (or even the China Democratic Socialist Party, one of its components) replace the GMD as the ROC's ruling party, though how that would come about (in a manner that is plausible) I don't know. However, there's a range of possibilities to approaching the GMD, even slowing down the Northern Expedition so that the GMD could more thoroughly remove the warlords from Chinese political life (hence making the Northern Expedition seem like a Chinese version of the Mexican Revolution). Even better here would be if the UK supported the GMD's efforts at unifying China from the beginning, way back in the 1920s (after all, HK was one of the birthplaces of the GMD, and Sun Yat-sen spent some time in the city and wished something like its administrative system could be replicated in China), so the GMD would not be totally reliant on the Soviet Union for, well, everything. Now, of course, there would have to be a price for that support (most likely including submitting HK to GMD authority, though with some concessions to assuage the population and retain the civil service/administrative structure), but if that's what it takes to get an ally in China, then Britain would definitely try that if it's cheaper than continuing to maintain a direct colonial administration and would keep the ROC happy.
Of course, everything
@CalBear and
@Admiral Bloonbeard stated I also agree with, and does a better job explaining OTL than I could. But ultimately it comes down to the fact that no matter which Chinese government is in power, they will all want HK back under its rule, sooner rather than later. To many Chinese nationalists (including, it should be pointed out, many Hong Kongers as well, since they were tired of all the racism and all that from the colonial government), HK stood out as a sore thumb, a symbol of all that was wrong about the century of humiliation as
the foreign concession par excellence. Therefore, it was imperative that the last remnants of colonialism, like HK, should be dismantled and brought back to Chinese rule. It also was convenient that Britain didn't see much value in HK except in a transactional sense, as an entrepôt pointing the direction to the Chinese market, but run as cheaply as possible so as to not place a heavy burden on the public purse. Once HK stops being useful to Britain, than Britain will want to dispose of it at the earliest possible opportunity. IOTL, since Britain recognized the PRC early on as the legitimate government of China, that meant eventually following the PRC's timetable on this (and the PRC were willing to wait until the New Territories lease ran out). ITTL, if the ROC still managed a formidable presence on China itself, in some capacity, it will be much more impatient about HK, regardless of the lease, and would want it returned ASAP. If it was handed over to the GMD as a confidence-building measure early on, that would be a great start to a more successful Northern Expedition ITTL.
