You don't need a GP to sign off, but you'll probably need a couple of prescriptions from one. Your midwife will probably know one if yours is unwilling. My GP read my midwife's letter, was delighted to see I wasn't using a private ob and did exactly as the letter asked.

Some IMs can prescribe. A couple in my midwife's practice are able to now (had to complete a course and I think also have practicing rights at a hospital, from memory).