I can't speak for the rarity in the UK, but they're definitely not that hard to find, good dual purpose birds stateside. They lay reliably and frequently, usually they don't slow down in winter either.
I checked it out and you can buy week old some chicks for 8 Euro each from perfect poultry in 4 different colors. I don't know if this is tremendously expensive for chicks there (would be here), but that seems to be their flat rate for any breed.