The honest answer is finding a breeder is all on you.
Sorry if that sounds harsh, but I think finding a breeder is like finding a partner/husband/wife - it's an incredibly personal choice. It can take months to find a decent breeder for the breed you want - even really common breeds like cocker spaniels and cavaliers - and then months to wait for a litter.
Someone could post on this thread and say they've found a breeder, and you'll think 'oh, well that person recommends this breeder, that must mean they're good.' But, 99.9% of the time, the puppies that person is breeding will not be right for you, or you won't be happy with the health tests etc. People pop up all the times on mumsnet, saying they've found a good breeder of XYZ breed but actually when you dig deeper, it turns out it was a puppy farmer, or no health tests were done, or there were huge glowing red flags.
Your best starting point should be your local breed club, followed by cross-checking the breeders on Champ Dogs and the KC website (which is the only way to be 100% sure of the health tests going back generations). Even then, you need to be careful as the local breed club will, sometimes, recommend breeders who are completely inappropriate (there was a poster a while ago who found a KC registered, breed club recommended, golden breeder. It turned out to be a puppy farmer).
Honestly, finding a good breeder or designer doodles/crossbreeds is incredibly hard - and part of that is exactly what @GelatinousDynamo said. Good breeders, with the healthiest dogs, do not produce mongrels (for lack of a better term) because they are breeding to make the breed they own better. I like to think I am a good example of that - I breed Golden Retrievers. I have a boy at the moment, excellent temperament, health scores, pedigree etc. I could make an absolute fortune if I wanted to, studding him to standard poodle owners to produce Goldendoodles. But I won't (for many reasons, but part of that is because I breed to make my breed better).
You really would be better off trying for a cocker spaniel, show lines, because there are hundreds of breeders in the UK, it'll be easier to check generational health tests and much easier to vet them.