For me the major loss is the recruitment - if that can be relocated to Britannia, Hispania and Italia
As for what Spain and Italy are concerned, most of the ressources avaible there were almsot entierly used up in local matters already historically. There wasn't much more to take from.
As for Brittania, even disregarding that with Gaul gone, it can't realistically be expected to see anything managing to coming up to whatever remains of the Empire, we're talking of an underdevelloped, under-inhabited and over-militarized part of the Empire already. It's either they take from Britain whatever they can as IOTL, either they let it only to kiss them farewell.
I'll stress this : by the Vth, every place was dried out of reinforcements, and giving the increasing pressure over limes then within Romania itself, it wasn't nearly enough to deal with everything. Giving that Britain was, as far as WRE was concerned, fairly unimportant, it was dried out and leaved to itself waiting for an hypothetical better time. There's no way that the lack of ressources an abandoment of Gaul would be compensated, especially not by an isolated Britain.
I'd be intending to fund it through having fewer fortifications than the Gallic Limes required.
The Gallic limes was fairly already relatively light, compared to Danube, mostly because it was relatively safer.
If something,
if we go trough the theoritical organisation of Vth Roman armies, Britain really shows up as having a more militarized (and more or less ineffectual) limes than Gaul. You'd notice as well the absence of significant arsenal in Roman Britain compared to Gaul, which would have meant abandoning not only territory but equipment and military structures.
I can't make it simpler than, militarily, abandoning Gaul in favour of Britain would have meant abandoning both for the sake allowing Barbarians roaming more freely in Romania.
It makes, absolutely, irremediably, entierly, irrevocably, no military or political sense whatsoever.
Do I have the numbers - no, so it would only make sense from a purely fiscal perspective if the savings were large enough
Saving were in Rome, not in the thin network of Britto-Roman cities that didn't managed to protect themselves as one entity when let to their own (pretty much thin themselves) resources. And for what matter paying off the early Vth century Roman army there, it's as we expected (all proportions kept, of course) Afghanistan to pay up for all the expense of Roman army in the country.
I know this is hard to swallow culturally for some people issued from the english-speaking world, but Roman Britain was the less develloped, structured and wealthy of Roman provinces. Period.
Ideally I'd prefer to see my scenario implemented earlier so that the Saxons don't have a power base in Britannia, but that would be down to exactly when this idea would be implemented
It's less about power-bases existing right from the beggining, than laeti and foedi tended to be such after a while and the degradation of Roman authority in the Vth, with leaders generally emerging out of it and mixing up various foedi (but as well provincial) assets. Probably what happened with Cerdic of Wessex which may have been the result of such mixing-up with native leaders going barbarized (if the name of Cerdic is any indication, giving it may be more Brittonic than Germanic).
By the mid-IVth century, in the wake of Pictish/Gaelic/Germanic raids in Britain, you certainly already had a Germanic presence in the Saxon Shores (both side of the Channel).
That said, remember that the ethnogenesis of Barbarian peoples wasn't yet that fixed : most of what ended to be called "Saxon", "Angles" or "Jutes" tended to be a mix of various groups that came at different periods.
For instance, Kentish Jutes may (probably IMO) be a mix of Gaul's, Old Saxony's and Peninsular (possibly on the lead) Jutes driven by the general North Sea moves (themselves driven by Danes and the general disorder in Scandinavia), mixing up with whatever (close enough or not) human groups they went trough, as Frisii.
Generally, Barbarian ethnogenesis involves several waves of settlement and mixng-up with more or less romanized Barbarians or Roman groups, to form a distinct identity after a while.
So even if you manage to nerf Saxons already settled in Britain in the early Vth, they'd likely end up swallowed (or swallowing up as it might have been the case in Wessex) whatever groups add new human layers.
Long story short, it's less about power-bases being already there, than the potential for becoming these (thanks to a military edge) during the collapse of western Roman state.
Implementing a system of fortified towns in the south east and other exposed targets as per Alfred the Great should at least be sufficient
The existing fortifications of the IVth are often considered to have been a disproportioned expense of ressources, tough. Nobody could pay more for this, and by the late IVth, Roman army was already regrouping to the South-East (abandoning the western shores to Gaels, and eventually Hadrian's wall to Picts).
You did have IOTL a re-centering of Roman army in Britain around the south-east. It's just that the historical situations points how it simply wasn't viable.
the only way Britannia could function is with a huge amount of autonomy, with finances and reinforcements sent from Hispania.
But why Rome would have given even more ressources (financial or military) when they didn't have enough already? I'm not picky, I just need one reason why Ravenna would think it would be a good idea to sacrifice the region next-door to your capital that still barely worked out as a military border for the sake of keeping a part of an overly-expensive army in Britain where it would sit all day on their fingers for all the emperor could matter with.
Admitting that they're somehow convinced it's the only right thing to do, regardless of the reason, how could they do so? Classis Britannica and Classis Germanica disappeared by the IIIrd century (probably replaced by a decentralized nvary in the Shore), and you never had that of a noticable naval roman presence in western Spain. How come a WRE that is essentially dependent on ERE to anything remotely tied to seafare could pull an exarchate?
I'm confused, so the Empire can defend the Eastern Alps and the Rhine perfectly fine, but can't fortify/defend the Pyrenees and Western Alps instead?
I strongly advise you to read more carfully posts you're answering.
I specifically said that the Rhine limes was
relatively safer, compared to other European limes (especially Danube, but the crumbling limes in Britannia could fairly count) in spite of the events of 407. I also said that eastern Alps were the traditional way of invasion, giving Illyricum was the soft-underbelly, strategically speaking, of Late Romania.
It doesn't mean at the latest "perfectly fine".
Anyway.
No they can't fortify western Alps or Pyrenees, because they had not the ressources to do so : they barely had enough resources (or unity) to not crumble overnight after the death of Honorius, which is why they were more and more reliant over foederati to ensure the security of some regions, as Franks in Toxandria or Saxons in the Channel.
Giving that Barbarians formed more and more of the defensive strategy of the empire (
it became painfully obvious with Aetius management of the Catalunic Fields), it would have make no sense attempting (out of an empty treasury) to create new limes against Barbarians if you had to man them with foederati eventually.
I can't make it simpler as : you don't undergo a wide promgram of fortifications while you're being pressured, raided and invaded in all parts of your territory and where you barely manage to pay (when you manage it, which becomes harder each time) your already existing forces, critically in places where it's still failing the lest at its job.
Interestingly the way you describe that just leads me to think that the best Roman solution in the West would always have been to go on the offensive, paying the Foederati with German land
They tried that (notably with Sarmatians peoples; but also germanic peoples such as during Justin's campaigns in Gaul) : it failed to be much of a viable alternative by the IVth with hunnic pressure, climatic change, need for military and rural production manpower,prestige and attraction of core Romania...
It doesn't seems to have been that favoured compared to laeti of the Principate and Dominate, or small-scale foedi eventually.