Go and stay in a lodge/log cabin in Cornwall.
Check out
http://www.hoseasons.co.uk/lodges/ as they have loads on there. You can go either Monday-Friday or Friday-Monday and you could get one with a hot tub, well within that £500 price range.
I'm sure you'll find something you like on there, but if not, then have a look at
http://www.forestholidays.co.uk/cabins.aspx
It's exactly the same kind of thing, but just putting that on there incase you can't find the dates you want on Hoseasons.
Have a great time! :-)