Wales
Hotel Portmeirion is in the middle of a crazy fantasy village http://www.portmeirion-village.com/ and Sergeants used to be a police station http://www.sergeantsrestaurant.com/

England
Hotel Pelirocco has rock and roll themed bedrooms www.hotelpelirocco.co.uk and the Three Ways House Hotel has the Pudding Club http://www.puddingclub.com/

Scotland
You can stay in a 19th century church http://www.adelaides.co.uk/ or a lighthouse http://www.lighthousehotel.co.uk/index.html

For unusual historic properties also take a look at www.nationaltrust.org.uk and www.landmarktrust.org.uk and http://www.english-heritage.org.uk/