Fast trains take around 90 minutes to do London-Bristol
Walk up one way fare is £48 ( £62.50 before 9.30 on a weekday) - Trains in the UK are very expensive in general
You can get much cheaper fares booking in advance for specific trains (tickets tie you to a particular depature time and aren't valid on other trains and are non refundable normally) - Fares start at about £15 one way for these advance purchase tickets ... the cheapest fares are to be had if booked well in advance (at least a week) and at less busy times.