Just had a week away renting a small castle in Exeter.
It was only something like £400 for a 4 night break Monday-Friday and they use it at weekends for weddings. I think they are fairly busy for bookings, but if you are thinking of a romantic break or need to win some brownie points from the Mrs, it is well worth booking up. It is a self contained apartment with a good kitchen and bathroom etc. It has it's own grounds although the public can walk in the grounds and it was also within traveling distance of several sea side resorts like Exmouth and Teignmouth.
If anyone is interested further details below:
http://www.haldonbelvedere.co.uk/Haldon_Belvedere_.html