I think a bid to income is better for overall game balance than a bid for preplacement units. In my experience if you award a higher amount of extra cash to the underdog but restrict it to normal purchasing, you take the emphasis off the first round battles which seems to work well for both sides being more satisfied.
Or just go with some set bid that doesn’t bust the opening round. In most of the older boards you could usually fix things by giving Russia an extra fighter at Moscow. That was my preference in AA50, even if there were “better bids” for Allies, that one usually was enough to keep players happy without being overly distorting.
Another way is to just give the underdog side a +15 ipc bonus to starting income, and let them choose how to split it up between the individual powers on their team.
The reason I don’t like preplacement bids for units is because players invariably use them in the breaker battles, to swing the TUV where it can have the most impact. And for whatever reason that always seems to mean a busted Med/Egypt game in every version of A&A ever haha!
If you keep it to income only then you don’t have to muddle through those, and it also gives a better sense of what is really necessary for ipcs to bid a balanced game. Usually this amount is higher than but most boards can be brought into balance somewhere in the 15-20 ipcs range on income.
But yeah, otherwise UK sub, since it gives you a much better chance on a heavy TUV swing, and UK is the first allied power that can make an attack.