In a nutshell, 6 hours I believe is the maximum time you can get credit for if you have a PPL(A). I think it's only the Principles of Flight exam at PPL that you would have to sit.
Cost? Depends on type but somewhere between £200 and £250 per hour but bear in mind you might not pass in 39 hours.
You could try FAST Helicopters at Thruxton, HeliAir at Wellesbourne or Sloanes at Sywell.
Cheers
Whirls