They are not fighting each other. But some of the FSA groups such as the Al Nusra Front and the Ahrar Al Sham are having issues with the Kurdish groups. Other than that, they are united in their fight against the Syrian government. They have some-sort of an understanding with the ISIS, and many of the rebel fighters travel to Raqqa to receive training from the ISIS. The UK government can use them to fight against Assad (if they are not already doing that), but they are not going to fight the ISIS.