Everyone travelling by train from Temple Meads and Parkway should be aware that queueing system will be in operation for boarding trains. Thus you may not end up on the train you booked on. So get to station in plenty of time. Also same for trains back from Paddington, I believe.
You won't get on the trains coming from Bristol they will be full, or if your lucky enough to get on one you could be sat next to Red Boycie Robin
If you are driving to Reading you may as well go on to a tube station on a tube line north east of Wembley.