07:04 Westbury to Cheltenham Spa - through train, except this morning where it's a bus from Swindon. Customer joins the train at Westbury to travel through to Cheltenham Spa - fair enough.
Since he was travelling all the way to Cheltenham, perhaps the information screens/announcements at Westbury could have suggested he travelled via Bristol on the 0709 instead to avoid changing onto the replacement bus. He might even have made it onto the 0800 from Bristol and arrived earlier than expected.