Hardly any medical schools in the UK require more than 2 sciences at A Level, ie, they want just Chem and Biology.
However, it may be a good idea to take maths to AS level as this could help with any aptitude tests (UKCAT and/or BMAT) you may have to take. Both these tests require maths to GCSE level but taking maths as a subject in Year 12 will keep your maths skills going and make this a potentially easier process.
That being said, maths AS isn't at all compulsory, universities don't generally 'look' for it in their candidates.
I'm in the process of choosing whether to take Maths for A2 next year instead of Theatre Studies. We shall see on the 18th