I've applied for maths and everywhere I looked, not doing further maths wasn't an issue because they know that a lot of schools don't offer it, and they've all said that anything you need to know, you'll catch up with, so don't worry about that.
If you don't get any offers for medicine, can't you decline your chemistry offers and then use the Extra thing on UCAS to apply somewhere for maths? I think you can only apply to one uni at a time so you wouldn't get an insurance choice, but it's probably easier than trying to go through clearing.