Talking of being stuck in the middle. I live in Leamington Spa and have a similar problem to you (although I do drive)
Unbelievably the closest beach that gets any semi good waves is Bournemouth which is 2 hrs on a good run and is therefore possible as a day trip (your mum can probably find something to do here while you're in the water.) They are also planning to build an artificial reef at Boscombe pier in Bournemouth which could make the waves better. The only problem is this gets crowded at the weekend because it is also close for the London lot!.
Next closest is probably Rest Bay near Porthcawl, the other side of Cardiff - it's about 2 1/2 hours so can also be a day trip. It faces almost west, so probably gets some good swell. I went all the way there once and it was flat so I haven't actually surfed here, but there a pictures of good waves here on the internet.
Next you're looking at either the Gower or West Wales - Borth looks like it can be good.
Check out this couple of sites:
forces-of-nature.co.uk - based in West Wales but has a real good community vibe and everyone's really friendly. There's also a beach guide and some photos.
beachwizard.com - has info posted by surfers about all beaches in Europe. - there are some pics of Rest bay on here.
Hope this helps, if you want any more advice/ help my mail address is email@example.com