Best answer by stephenrand
The sad news is that Pure Planet don't currently take on customers with prepayment meters...
Which is a real shame.
Citizens Advice say "Get your prepayment meter replaced with a normal meter. You can ask your energy supplier to replace your prepayment meter with a normal meter, as long as you're not in debt to them. Replacing the meter will let you pay for your gas or electricity after you use it, rather than in advance."
So perhaps you could get EDF to do that asap, then you can switch to Pure Planet!?