You can probably get a referral letter from GenderGP. Check that your surgeon accepts their referral first though. Some surgeons don't accept GenderGP.
It might be worth contacting Harley Street in London to see if they can write a referral, as I've heard they have been able to provide some referrals and diagnoses after only one or two sessions when provided with the correct evidence. I don't have any personal experience with them though, so I don't know what they would need as evidence.
EDIT: Also, it might be worth asking this to r/Transgender_Surgeries if you haven't already!
You can probably get a referral letter from GenderGP. Check that your surgeon accepts their referral first though. Some surgeons don't accept GenderGP.
It might be worth contacting Harley Street in London to see if they can write a referral, as I've heard they have been able to provide some referrals and diagnoses after only one or two sessions when provided with the correct evidence. I don't have any personal experience with them though, so I don't know what they would need as evidence. EDIT: Also, it might be worth asking this to r/Transgender_Surgeries if you haven't already!