I'm teching myself the 6 extra modules this year (Or at least thats the *plan*!)
Alot of the applied modules are really very easy, S2 is easy and M2 is a followon from physics. Not only that but the text books are good and self-teaching is not hard.
You can do an extra applied module so you can swap marks if you do better in one than another, but if you're predicted an A you don't really need to.
I dropped a subject this year (i;m in U6th) so have the time to teach myself an A-Level but the FP's are full of new concepts which although are not very hard, it takes time to come to terms with them and understand them, and be able to totally explain what they are in your own language.