I'm not sure of the current population of the UK, but the UK Ring of the IBM is about 1600.
You also have to look at the terms magician and layman. Layman is somone with little or no magic knowledge. Magician can vary, from an experienced magician, to someone with an amateur knowledge, to someone who knows two card tricks that he learnt off a mate while serving in the forces and bores the ar*e off his grandkids.
I would have guessed that the ratio is somewhere like 1 magician in 50,000. So now I will ready for the backlash
