Manchester notionally wants a 3.0 GPA, but that’s mostly a sop to American students who keep insisting on asking about GPAs!
You only write 1 essay - the Personal Statement (PS), that goes to all the places you apply. The focus of the essay is on why you & the Classics program are a good fit.
As posted above, as a U$ payer, if you meet the reqs & make a decent fist of the PS & LoR you will most likely get an offer. There are some courses- IR at StAs for example- that are a bit tougher, b/c they are so oversubscribed, but if you are genuinely interested in classics, and have done the homework to know how to focus your PS, you should be fine- except for Oxbridge, obviously.
Some UK universities do accept the Common App, but you can only apply to UK unis through Common App OR UCAS (you can’t mix’n’match) so if there are places that you want to apply to that aren’t on the Common App, you have to go UCAS. You get 5 choices on UCAS.
@tkoparent is correct: every UK uni has the “typical” offer posted online, special requirements by course, and international equivalents,