I'm not subject to the NATS regulations but we have a more or less equivalent system.
Your "Visual Performance" is not based on your refractive error alone (+/-), but is a combination of refraction, astigmatism, visual acuity, colour vision & the combined balance of those errors..
I think we require between +3/-5, astigmatisim <+/-3(both eyes within 2 of each other), acuity 6/6 combined - corrected if necessary - only 1 pair of glasses may be used, one eye can be 6/9, normal colour, normal fields, normal stereopsis.
In my facility, we have a complicated visual performance test every year, and are sent for an opthalmological examination every 3 yrs (or every year if you wear glasses)
If you can see okay to drive and live a normal life, and you're not prone to walking into doors and tables that you just didn't see, then you'll probably be fine. The testing is as much for your safety as for the aircrew + pax.