sod ums. an A is an A, and if you achieve one overall then well done. besides, individual modules can come in such weirdly marked batches that i wouldn't even worry about it. if you do get a really high ums mark in an individual module or the subject as a whole, then great but it's not really essential.
exams are also a different ball game to uni assignments, in my opinion. although you will have uni exams, they should hopefully be a bit better than a levels.
personally, i wouldn't have cared if i got an E in an A level module, as long as the end result was what i needed overall. i mean, i suppose i'd be a bit upset with regards to the odd relationship between grade and work put in, but don't let it worry you. just focus on that 480/600 at the end, or whatever you need out of 300 for A2. actually... not for english, but i did get an E in a A2 module (latin language)! found it a bit annoying, but with other modules of higher marks, i ended up with the overall grade i needed and just laughed it off.
seriously, by the time you're strolling around whichever uni you get into in less that a year's time, you could probably use your A level exam certificates as loo paper. (ok, well you might need them for registration or something but you know what i mean). it's a completely fresh start, so if you performed a bit less than expected in a random AS level module, you've got all the time, facilities and teaching to really improve in that area. besdies, there's a whole load of new topics and texts to explore. i'm sure you'll be absolutely fine. also; as you said, you're sure you'll enjoy it and that's possible the best qualification for this course, or any one for that matter. good luck in your application.