Um... are you sure? Boilers start at minimum £400 from all the prices I can find, and that doesn't include delivery, installation, or stuff like if you have a modern high pressure boiler installed then the radiators may need re-doing, or you might need a new electricity supply for it, or change of hot water tank or, or...

If you know of somewhere that will sells and install a new boiler for £200, PLEASE share your secret!