I would advise taking the megabus rather than the train.
The reason?
The train ticket without a railcard will cost you £60, but with a railcard it will also cost you £60 (£20 for the card but still £40 for the ticket).
However, you can take the megabus for £1 each way (if you book now) plus a 50p booking charge, which will cost you £2.50 in total for the round trip.
The only drawback is that the megabus takes 4.5 hours, compared to 2.5hrs on the train, but it is really worth paying an extra £57 just to get there 2 hours earlier, especially if you are not travelling back to Manchester on the same day?
To put it into perspective, if you travelled to London on a National Express bus, it would cost you about £30 and the journey could take up to 6hrs each way (with a possible change in Birmingham or Milton Keynes).