Newblood, its the same with any registration scheme (Corgi,Hetas,Oftec). Get the qualification that proves your competant and then fill out the application form. all info is avaliable through the solar trade association web site.Just like every one else they will periodically inspect your work and carry out an initial assement.
With the BPEC qual your card is issued by the STA, why im not sure maybe silverback can answer that one? and what form of membership that offers im again not sure?
Alot of the large firms are charging alot of money but i suppose their overheads are higher, near us in Notts and Lincoln £6000-8000 isnt un heard of, and thats not including Smart Energy's prices.