Avoid Valentine's Day, bank holidays etc. if you want a good deal; you need to pick times when the hotel will be quiet. We had one Wednesday night in our nearest Malmaison (half-an-hour's train ride) to celebrate the King's birthday last March. When checking in, we made it known it was the King's birthday and got a free room upgrade! Book a posh room last-minute or using any special offers your favourite hotels are running (check on their website and sign up to their mailing list to receive details of these).
Breakfast as late as possible so that it essentially becomes brunch. Order something filling like a Full English, and there will be no need to buy lunch. Help yourself to a piece of fruit from the fruit bowl before you return to your room, and keep that as a mid-afternoon snack. Do not use the mini-bar and don't be afraid to ask for more tea and coffee in your room.
When picking a hotel, don't go too far afield, as you will waste time on the M6. Plus, your kindly babysitters will be that little bit more relaxed if they know you are in the same time-zone.
All you need is to dip your toe in the waters of luxury, and if you aren't worrying about the hotel bill, you'll enjoy it all the more. Our one night in heaven was worth every penny and we came home totally relaxed and full of plans to convert our loft into a master bedroom and ensuite!