That is a very popular course at Imperial because they are certainly one of the best in Europe for it... Hence they are asking for AAA at A-level. Althoguh if you happend to drop DT at the end of the yr...i don't think u'd get an offer of AAA offer inc AVCE Performing Arts.
If i were you, I'd do Maths, Further Maths, Physics as this would increase ur chance of getting an Offer. You will have to learn the maths anyway whilst at imperial so you might as well learn it now and increase ur chances of getting an offer.