I am going to also fly in the face of convention and suggest Milton Keynes!
A great shopping centre, all under one, clean roof with lots of restaurants and coffee shops. Indoor skiing/snowboarding and skydiving - for the active amongst us. A very successful theatre. And still in town, lovely canal walks, cycle paths and Bletchley Park.
Within half an hour's drive we have a number of NT treasures - Stowe, Waddeson Manor, Ascott House, Claydon House and Canon's Ashby. The first two are huge sparkly jewels in the NT crown and the others minor gems. Not forgetting Woburn just down the road, with safari park, deer park and the Abbey. Be astounded by the fact that they have hung onto the property and paintings etc since Tudor times. Only the queen has more Canalettos.