I was thinking Physics, Biology, and Maths but I know Further Maths is highly prized in engineering. There is also Chemistry to think about.
What do you think? Those 3? Add a 4th? Or a different combination?
I'd say that Physics, Biology and Maths are a pretty solid combination for biomedical engineering, although most of the people that I know who have gone on to study biomedical engineering started off studying the three sciences and maths at A Level.