Further maths is definitely the best option. The material from further maths is taught in the first year mathematics course (for Cambridge at least), but the pace is very quick and I could imagine it being significantly more difficult if you had not met the material before. Physics degrees are very mathematical so you will want as much proficiency with mathematics as possible.
Claree is right about a lack of chemistry limiting you 1A Natural Sciences options to an extent, but you could conceivably take options like materials science or earth science without too much trouble.