Lizz, you can get the train from Manchester to London (it takes about 2 hours, 20 minutes), and it can work out cheaper if you buy two single tickets, rather than one return. Singles start at £12.50. You can look up on www.thetrainline.co.uk
If you were to get the National express, it takes about 4 hours, 30 (minutes (direct), and fares vary greatly. You can get the special fares, so it can work out as cheap as maybe £10-£15 return, but there are certain restrictions I believe, so it's definitely worth checking out the website!
It all depends on whether you want to spend 4-5 hours on a bus!