The best way to cut the SFSG would be to cut the RAF Regt unit and Royal Marines and just concentrate on 1 para which is probably going to be cut back down to around 400 men anyway.
No idea on the RAF Regt but a cut to RM or PARA? I only knew PARA as animals and RM as intelligent and capable people. However, I would say fund both units. They both have their place and provide solid blokes to the upper echelon.