Welcome to Frog Forum
Sadly I know not of any breeders in the UK, but there are some experienced breeders on here as well as many experienced keepers.
This will get you started on their basic care. Frog Forum - Pacman and Horned Frogs - Ceratophrys - Care and Breeding
If you buy a baby they are best kept in medium to large critter keepers. Buy a UTH(Under Tank Heater) for Hermit Crabs since they are the only UTH safe for plastic and place it on the side of the keeper. These are burrowing frogs and burrow to thermoregulate their body temps and if they burrow to cool off over the UTH they wont understand why they cannot and may overheat and die. Daytime temps are 80° F to 83°F and night time temps 75°F to 79°F, but hover around 78°F. Humidity should be 75% to 80%. They're nocturnal so it is suggested to feed them at night.
Any further questions feel free to ask.