Not ANYTHING... there are courses at some schools which require 2 Sciences aside from Maths, such as Medicine/Dentistry or Materials/Chemical/Petroleum Engineering. In any case, requirements aside, the Sciences work best when they complement each other.

Short of Mathematics or Medicine/Dentistry at some schools, and Biological Sciences, your combination is good enough for most courses, including Economics, Finance, Accounting, Business, Law, Engineering, Physics, Chemistry, Pharmacy (excluding vocational 'Arts' like English, Design, Music etc).

With Maths, Economics, and Accounting, you'll obviously not be eligible for Science courses at most schools.

I would advise against Accounting as it is never a prerequisite for good schools and is not appreciated at A Level (compared to Economics/Mathematics/Science).

BTW for the "every route possible" way to happen you need to take 5 which includes Further Mathematics, because only then will you be able to choose Mathematics and its joint degrees at every school.

Precisely why would you have a hard time? Are you suggesting that if you took Science i/o Accounting and Economics, you'll have difficulties with Economics or Actuarial Science at the top schools? It's no surprise that Accounting is not amongst the list or preferred or perfectly acceptable subjects for these "top unis", in fact it is quite the opposite. Furthermore, these "top unis" would rather have Geography (perhaps not really a Science), Biology, Physics, or Chemistry over Economics where available (i'm not suggesting preference is an indication of success).

Biology is most definitely a partial requirement to do Medicine for some universities and/or their colleges. I say partial because in many instances you only need AS Biology if you have A Level Chemistry, but you may need A Level Biology if you only have AS Chemistry. All this means is not taking Biology severely limits your opportunities as the top/best schools are amongst those that need Biology. There's no harm taking Maths + Chemistry + Physics + Biology if you want to leave Medicine open but are sure you won't want to do Mathematics at a top school. Medicine almost always needs at least 2 Sciences at the top schools, and prefers Sciences and Maths over non-Sciences after the specific subject requirements (2/3 Sciences including Chemistry and Biology, either AS/A Level) are met.