How about the coach? Look at yellow pages and you should find a company - maybe National Express. The last time I went to London on a coach (about 18 months ago!) it cost under a tenner.
This one only goes as far as Cardiff but it really is very cheap (advertised as �1 but invariably costs a bit more - something like �10) http://www.megabus.com/
A return from Swansea to Cardiff on Train is less than �10 also.