Honestly there's no course that requires computing that you wouldn't already be eligible for with maths/further maths - so I'd take biology as that opens up more doors for you IMO, plus having just finished it at A2 I really liked it, but that's just me. Economics is obviously a solid choice considering you're interested in going into finance.
There's also the point that many universities see computing as a 'soft' A level, especially compared to a pure science like Biology. There's nothing stopping you pursuing programming in your own time - hypothetically if you ended up wanting to go into computer science you'd: a) start from scratch so computing would give you an advantage but ultimately is irrelevant b) already be more than qualified