Dan,
Go for it mate. I am a rigger 31 years old this month and have OASC on the 10th July for NCA selection.
You dont have to do A levels for NCA, 5 GCSE's including maths and a science. Speak to PSF there is a pack they will give you with various forms and go to the careers office as this is where your application is processed. It can take a while for all of the paperwork to get sorted and you have to get manning clearance from you trade also. As for upper age limit 32 for NCA. Ring around some units and go and visit them, in the last 12 months i have been to 4 different stations and had a superb time at each getting advice and flying. 55 at cranwell also do a famil visit on the last thursday of each month, well worth it.
Hope this helps mate.