Obviously a top MMA fighter.
Hand to hand combat is a small part of sas training compared to weapons training, CQB, Fibua, hostage situations, explosives and all the rest.
Also sas guys tend to be more endurance types than super strong because running around hills with 50kg on your back
Whereas an MMA fighter is devoted to his 20 minutes in the ring or whatever it is. He'll almost certainly win.