Near London Compromise Hotel Location

We have a three day hold on a cabin on a TA cruise in April that terminates at Southhampton. This was a very spur of the moment decision so we have not had time to research much yet. Budget (time & money) is very constrained but as long as we are in London area we would like to spend two nights and have a whole day+ to see a few sights.

Is there a good compromise location on the tube that offers lower hotel costs than central London but not really bad commute time? Our touring day would be a Sunday so that part would be less crowded. Perferably somewhere the National Express bus serves. Added factor, my wife uses a wheelchair (which I push) for anything more than short distances so accessability (ramps, lifts, etc...) would be good.

Thanks!