Luton Hoo and Grayshotts are fantastic as there's so much to do and see at both places. Also, anywhere in London is a good choice as there's shopping, restaurants, theatres etc etc - can't go wrong! Best thing to do is find somewhere you love on here, note the price and then call up the venue to book :)
Champneys is a great place to go on your own. There is a table at dinner time where you can sit with other guests who have come on there own, if you want .they do good offers to. Your never bored with lotsof classes and treatments to have. Hope this helps.