I assume this is nothing to do with BMI?
I wouldn't necessarily put those chains you mentioned into luxury break territory! The Luxury Hotels forum on FT is a good resource if you never normally read that. Fundamentally, there are under 10 decent weekend break-type properties in the UK if you are interested in serious luxury.
I presume one of the hotel booking engines like Expedia would let you search for a 5* within 50 miles of, say, somewhere in the Cotswolds. Mr & Mrs Smith website has a lot of UK high-end weekend-away properties on it, worth a look but again not sure if you can search 'everything' by date.