Hi Liam,
Another approach is to try for one of the Territorial Army, SAS squadrons, namely 21/23 SAS, depending on where you are located,just go to MOD web site and search UKSF Reserve.Its worth a try as Civilians can apply as well as serving (TA) or ex regular. Forget the typical attitude that people have of the TA, you just have to look out in the Stan and elswhere (including this web site'SAS TA) and you'll find a TA soldier or three In answer to your querry, there has been, fairly young individuals pass selection through the reserve/TA way, but they have been at the top of the scale regarding fitness and mental attitude. I know of 2 fell runners/mountain marathon runners who did just that, not long ago,And also apply for 22 selection from SAS reserve (after a few years), but as johntaxi22 rightly advises,try Para Regt, which is hard in its own right, remember SAS members are from various cap badges....Getting experience and knowledge behind you and being worldy wise after a few years in does go a long way to helping get nearer your goal? If you still feel you want to after recruit training and life in whatever you join, then why not apply once you have tasted army life?
Regards,
Dave