Whilst there may be barriers at stations between Paddington and Bristol TM▸ served by high speed services, excepting some platforms at PAD» and all of Chippenham, there still should be regular checks on board. Barriers aren't intelligent enough to check someone is on the right train with an Advance, they can't check that someone is dumb-belling, they are of no use checking someone who has started at an unbarriered station and has interchanged at say, Reading or Didcot, they can't prevent someone travelling in 1st Class on a Standard ticket.
Agreed, I find it frustrating that I bother to pay to travel and nobody is there to check. It's no wonder so many people choose to evade their fares!