I heard a lot of people talking about mobile Abbotskerswell, but I don't think there are any in the UK at all!
However, you are allowed to slaughter "small" amount of poultry on farm - actually its upto 10,000 (TEN THOUSAND) per year. I don't think you are planning to do more than that at the moment

And this rule applies to people who want to sell their poultry, which was raised, slaughtered and processed on the same farm.
Google Richard Perkins (or YouTube) for advice on small scale poultry slaughter house. He has one, albeit in Sweden. However the rules are not much different here either.