I live in SW London. The obvious answer is Brighton. But Mum is feeling miserable and I think a day (or night or 2 in a B&B) would cheer her up. I am thinking deck chairs, fish n chips, bingo, amusement arcades. Just old fashioned non-fancy stuff and I will take her and Dad and the kids.
I really can't think of anywhere my side of the M4. Not too far, 3 hours tops.