I started a couple of Days Out on the 05.40 (I think!) 529 Bristol to Gloucester. I say 'I think' because it doesn't appear in my 1974-5 timetable; maybe it wasn't advertised because no-one in their right mind got up at that time in the morning... The driver would stop at the 'News Stop' newsagent in Horfield to pick up his paper, and have a chat and a fag.The 529 took just over two hours to get to Gloucester. As an alternative, you could have taken the 526 which went via Wotton and Dursley, was described as 'scenic' and took about half an hour longer.
There was a 419 Bristol to Swindon that left Bristol at 04.something, which I think I may have caught once, though I'm not sure how I got to the bus station in time as the first No.3 rolled into the Haymarket just before 6am. Maybe I walked?
Just had a look online. One website has a Time Table (TT) for the 418 Bristol to Swindon: first one from Bristol given as leaving at 0645 in the Time Table (TT) valid 15/06/1969, journey time on that was 2.5hrs.
IIRC▸ there was, until a few years ago, an X54 Bath - Swindon on Sundays and Bank Holidays.
I had never heard of the 526 though (too old to have done so I think: when did it last run?):first from Bristol was 0651hrs, journey time approx. 2hrs 20min.
Dave
Edit: VickiS - Clarifying abbreviation