Unfortunately because of family issues at that time I never finished my "A" levels but I got half way in Oxted School. I was studying, Biology, Chemistry, Math and Physics.
Those are ideal "A" levels for any medical or scientific career.
My advice would be that you should complete your "A" levels and consider a university degree course. You will find that difficult after 26 so you only have five years!
Graduates at 31 earn 26% more than non graduates and over a working life about £230,000 more!
Quite apart from the difficulty in getting training in plumbing there is severe difficulty in getting any jobs now and obviously those with prior experience will get those few jobs which may come up.
Tony