The Witchery by the Castle in Edinburgh is regularly voted Scotland' most romantic - it's totally decadent. It's in a 16th-century building up a private turret stair off the Royal Mile and each suite is completely over-the-top opulent with open log fire, huge rolltop bath, several seating areas, a separate dining room with full room service and every luxury you would want. Unforgettable.
The village of Minster Lovell in Oxfordshire has The Dove and another hotel across the road whose name I can't remember, but I think they are under the same management - a picture postcard village from where you can get to the Cotswolds and various other places easily - the hotels have exposed beams etc etc. I stayed there once and loved it.