It's fine, I'm happy to help.
If you're carrying all 4 to A2 and your predicted grades are AAAB, then you would need a very high UKCAT score. If you get predictions of A*AAB, your score can be lower but I don't know how much lower unfortunately.
As far as I know, your predicted grades count as tariff points. You can only count AS tariff points if you drop the subject and don't continue it to A2.
If you pick up another AS in your A2 year and you get predicted a B for it (to give you predictions of A*AABb), you would have 530 UCAS points and you would need around 690 on your UKCAT based on last year's stats. Your school would have to predict you a B on the AS you'd be self-teaching and it's also a lot of work on top of 4 A2s, so really think about it before you decide to do this. If you get 795 on your UKCAT, you would be fine too based on last year's stats, but I would still try to get better predictions just to be safe. If you don't end up picking up another AS, predicted grades of A*AAA would be alright with a UKCAT of probably between 730-750. My estimates might be off though, so check with someone else just to be sure. When is your UKCAT? Also, General Studies and Critical Thinking would both go towards the total tariff points even though they won't be part of your offer. As far as I can tell, anything that has tariff points can be used to increase your total.
http://www.ucas.com/how-it-all-works/explore-your-options/entry-requirements/tariff-tablesLook through that link and see if anything applies to you (dance, music, EPQ etc). I know music and EPQ tariff points would definitely be included in your tariff point total. If you see something you want to do or have done, email Barts and ask them if it is acceptable as part of the tariff total.
They state the following on their website:
UCAS tariff is made up of a number of academic and non-academic qualifications. Applicants are encouraged to review the list of ‘tariffable’ qualifications and must ensure that all qualifications they have taken and/or are taking are listed on their UCAS application in order to be taken into consideration. As well as AS and A2 levels, other examples of tariffable qualifications include the Cambridge Pre-U, the ‘Extended Project’, the International Baccalaureate, the Irish Leaving Certificate, and Music Examinations. We will calculate your tariff for all tariffable qualifications you list on your UCAS application including those that are not taken into consideration for meeting our minimum academic requirements e.g. An AS or A’level in General Studies or Critical Thinking.
If you can already play a musical instrument, take the Grade 6 exam. I'm not sure what they're like or how much work they require, but a Distinction gives you 15 extra points so maybe look into that.
Good luck!