Yep, Cats Protection if you have a nearby branch or if not, call the RSPCA and explain the situation. They will either direct you to one of their minor treatment surgeries (about £7 for a consultation, drugs at cost, and I think about £20 for a cat neuter) or will put you in touch with a local branch member who will issue a voucher for REDUCED neutering at a private surgery if there's no RSPCA one near you. Occasionally they will refer you directly to their hospital, depending on where you live, but AFAIK these are far and few between.
A friend has just gone through the RSPCA to get her cat neutered. She took mog to their minor treatment surgery, had a consultation and then mog was placed on a waiting list for surgery at our local Vet training school.
The PDSA used to have a similar system and presumably still do.
WRT private treatment, shop around for quotes if you can, provided of course that standard of care is the same. Often there's quite a difference in prices, even with vets within a mile or two of each other.
One last thing - RSPCA treatment for pets owned by those on benefits is a godsend but DON'T buy Frontline etc from them. It's much cheaper online, with PetMeds being the best ime (great, fast customer service too).
HTH