Stratford from Bristol

[Beethree]

Heading up to see Abba in Stratford next weekend, from Bristol.
Trains are stupidly expensive, about £80pp return. My girlfriend has suggested we take Megabus for about £10, I’m trying to find a better alternative than 3 hours on a bus but considerably cheaper than the train.
Happy to drive as far as required as will be staying overnight so no concerns ref drinking etc.
Any suggestions? I was thinking drive to West Drayton and then get the Elizabeth line?

[Piginapoke]

You could just park in Stratford Westfield. Secure and not too expensive