Cutting it too close? Bath to Edinburgh

Yes, you are pushing your luck.

What will you do if any of those what-ifs fail you?

Knowing the traffic on UK roads, I would say there is a fair chance that you may have a delayed return, which then means you miss the A4 and the flight.

One hour 40 should be enough time at the airport.

You will have a very late arrival at your accommodation in Edinburgh. Being a later rotation, there is a chance that the flight could be delayed.

The road between Bath and Bristol is very congested at that time of the day. My suggestion would be that you get a taxi from Bath to the airport, rather than use the bus, which is not an express service. The journey will be quicker, and the taxi will leave as soon as you get back from your tour. The taxi will also wait should your tour be delayed.

Tour round the Cotswolds and Stonehenge from Bath in one day would in itself be too ambitious for me.

Are you doing a Mad Max tour? I did one a few years ago that did Stonehenge, Avebury, Lacock and Castle Combe and it was a nice day out. I find this company very dependable (have done 2 tours with them) but there is occasionally heavy traffic in some of that area so I'd expect to be late back in to Bath. I think both times I was 15-30 minutes late returning. BOTH tours left exactly on the dot or a few minutes early so you definitely get your money's worth.

I'd not do a full day tour with a flight to catch that evening. They do have some half day tours that get back early afternoon depending on what your main interest is and what day of the week you are considering.

Actually I think you've got some good buffers built in here. A few caveats though - does the tour drop you off at or very close to where the airport bus departs? If you miss that connection would you be prepared to pay for a taxi for what would be a fairly long journey? Once you get to Bristol at that time of night an hour would honestly be fine so long as you are not checking luggage. It's not a huge airport.

IF it's a MadMax or Lion tour (or I think most tours) the drop off is the area right in front of the Abbey Hotel across the road from the Parade Gardens. It's probably a 10 minute walk to the Railway station or to the bus station next door to it. Maybe less if you really hoof it.

Also...another consideration - luggage? The vans for the tours are the small vans and there is no room for luggage. You can bring a day pack but you'd not be able to bring even a carry-on size suitcase.