UK weekend away ideas for October

[gregd]

Apologies for another "where to stay in the UK" type of thread. I turn 40 in October and me and the Mrs have managed to talk some friends into having the kids / dog for the weekend. This is a rare treat for us as we really struggle for babysitters with one set of grandparents in the US and the others too doddery to help! Any ideas for (ahem) romantic UK weekend destinations, preferably within 3 or 4 hours tops of Milton Keynes "in case of emergencies" (Mrs D's words, not mine.. I'd happily be off to Hawaii!!)? Ideally we're after a memorable hotel / self-catering place with stuff to do and great places to eat/drink nearby.

I'd budget up to £500 or so for accomodation, so we should get somewhere quite nice..