Surprisingly there was the occasional RM passing selection for Hereford, and I think I once saw an RAF Regt lad as well.
You're a bit out of date AA! Even before they created a unified UKSF selection, there were plenty of RAF reg and RM applicants to the old Desmonds selection course. A lot of RAF came via the RAF mountain rescue cadre, and most RM were narrow SBS failures but recommended for Reg (eg the bloke who fell down the cliff and broke his leg on the 80s TV show on SBS selection - he turned up later in 22).
With unified UKSF selection, there is an even wider spread of applicants, from all parts of the services. RM applicants outnumber Para now (Royal Marines is about three times bigger than 1,2,3 Para combined). SBS "selection" does not really exist any more in the strict sense of the word. Basically if you pass UKSF selection, you can either stay in the Desmonds, or if you like swimming and fancy redoing selection in a Klepper, you can attempt the SC3 course in order to join SBS. So perfectly possible for RAF Reg to end up in SBS nowadays. If you fail SC3, you have option to go back to Hereford.