Buying a Labradoodle pup from a reputable breeder is quite difficult, simply because there aren't that many ethical, decent breeders of them about. Epupz and similar sites are likely to be littered with ads, most of them from puppy farmers (who are extremely good at conning people, even those who think they know what to look for) and back yard breeders, most of whom will not have bothered with proper health testing or don't have the knowledge to breed and rear sound, healthy puppies.
You're right that a Labradoodle shouldn't cost more than the pure breeds involved, so around £600-£800, but you should expect one to cost around that. It costs a lot of money to rear a litter properly to weaning, not to mention time and good breeders know that pricing puppies cheaply can attract the wrong sort of buyer.
There are loads of Labradoodles in rescue, often puppies too. Getting one of these not only means it'll cost you less, but it means the rescue centre involved will home check you and make sure that you are getting the right dog, because that is all they care about. It also means you'll absolutely have lifelong support and help, although you should expect that from a good breeder anyway. The Labradoodle Trust site has a lot of info on what to look out for when buying a puppy.
https://labradoodletrust.com/puppies.html
I have to say though, most people that want a Labradoodle can get what they want in a dog from either a Labrador or Poodle and are then also more likely to know what they are getting. Labradoodles still vary an awful lot in terms of looks and temprement and you won't know what you're going to get until your puppy grows up.