Choosing a-levels : MarketingWatch
I wouldn't mind saying that Psychology would be of help, but I am not sure if it's too academic for marketing.
I don't think you need to know that much maths, unless you're doing a lot of stats (i.e. AS Level should be fine). I haven't seen a lot of maths used in marketing in practice.
Business should be ideal, and economics is good.
After your BSc, you might also want to check out the Chartered Institute of Marketing (CIM). I'm looking into it for the digital marketing diploma at Level 6.