Does anyone know how much it might cost, and hard it would be, to have a downstairs cloakroom installed in a house in the UK? I am looking at a property to buy and it has a fair sized area at the end of the kitchen which I would need to have made into a cloakroom (the door into it would be off the hallway not the kitchen!) I have no idea about this sort of thing though!

A great deal of your expense would be the rough in for the sewer drain. Your kitchen pipe would probably be able to supply fresh water but would need additional venting and a larger drain to include a toilet. I don't think I know enough about your situation to put a price on the job.