Hey there,
Did you apply last time around, or just do the entrance exams to practice for this year? You obviously shouldn't get rejected on the basis of age within reason (but most schools are looking for people to contribute after they graduate -- if you're approximately 40 you still have a good 15 years of contributions to make at a minimum).
715 on the UKCAT is good, you should have got you an interview at Warwick assuming a decent PS with good experiences. It obviously suggests that your personal statement maybe wasn't quite as good as it could have been. I recommend the Personal Statement Advice Service here on TSR, they really do an incredible job and it's usually a successful GEM applicant who reviews it
They tore mine to shreds initially, and though I didn't make it last year, on my feedback I was told the PS was excellent. With 715 and a good personal statement, with more voluntary experiences this year, you should be in with a good shout if you reapply.
Southampton is a shot in the dark for everyone -- they have the most cryptic admissions system I have ever seen, so it really is pot luck assuming you meet the cut-off.
It's a pity about your GAMSAT; you're in the same position as me as I scored worse on the last section but hammered the first two! Luckily if you do get a better score and meet the cut-off you are almost guaranteed an interview at SGUL bar saying you're a convicted murderer on your personal statement.
I'd wait for your GAMSAT result and see what you get. If it's good, apply to SGUL, Nottingham, Swansea, and then I'd recommend Warwick again as a fourth option just for the sheer number of places. I doubt you were rejected based on age as they're pretty liberal at Warwick, so it was almost certainly down to your personal statement.
Good luck!