See
http://www.caa.co.uk/docs/175/srg_fcl_gid15.pdf
To paraphrase, before
applying you must hold a PPL(A)+Night Rating
or a CPL(A) and must meet certain requirements (50 hours cross country, for example - GID15 describes in detail). I cannot see any restriction on when you could
start.
As for cost, I'd guess and Arm and a Leg. Or something like that.