I think it's a modern language at GCSE, not at AS.
Also, not all med schools require UKCAT or BMAT - Birmingham doesn't and there may be others.
I haven't looked at every med school, but all those I have investigated don't accept Further Maths as a third A level. I would think it is very high risk to take it, it really narrows your options and then you have to have the perfect profile for the few that might accept it. Also it leaves you no option to drop one of the other subjects if AS's don't go according to plan. My school, with a good record of getting students in to med school, encourages an essay subject for the third A level, or at the very least for the 4th AS. (I'm doing Maths, Chem, Bio and History)
If you are going ahead with FM why not take a 5th AS in an essay subject? Also, be wary if your school does the Maths A in year 12 and the FM A in year 13 - some unis won't accept A's taken in different years.