Pimlico to Morton-in-Marsh by train: Advice for best way to travel by train? By tube then train?

Wow.

My suggestion is you train to Morten in the Marsh the night before.

Book the 0750 train from London Paddington to Moreton-in-Marsh. Pay on the day for the tube from Pimlico to Paddington. Get to Pimlico tube by 0705.

They are actually doing advance fares from Paddington to Moreton I M again by then, for £13 each, whereas a walk up fare is £44.
The 0750 train arrives at 0917, which doesn't give much leeway for any line problems.
I might think about travelling an hour earlier, for my own ease of mind.

I agree that in these circumstances I’d prefer to travel up the night before.

As has been mentioned, your options are the Paddington train at 0750 or 0650, arriving at 0917 or 0821 respectively.

And prior to that, either Underground (Pimlico to Oxford Circus via the Victoria line then Bakerloo line to Paddington) taking about 20 minutes, or use the bus from Pimlico stop A as it takes 24 minutes (including the walk either end, although that's very brief).

For the train out of Paddington definitely get Advance tickets - your only down side is that ties you to that train. £13 for the Advance - otherwise the walk up switches to an Anytime ticket at £44.60. The earliness of your train out is reducing the options.

Whether you go for the 0750 or not will depend on where the tour meets - although it's not a big place and only takes about 15 minutes to walk from the station to anywhere in the town

I took a GoCotswolds tour that simply picked up passengers at the Moreton-in-Marsh train station. I didn't travel to Moreton from London that day, however. I'm way too risk-averse for that. I was spending a few nights in Oxford and just hopped over to Moreton from there.

The reason why I would say to, at best, catch the 0650 train in preference to the 0750, preferably go the night before is exemplified by my travels today.
This is absolutely not a typical day, but is not massively atypical either.
I have, or will have, used 11 trains today, covering a bit under 300 miles.
I have had-
a level crossing failure (no real delay thankfully);
a signal failure causing me to have to do a mad dash through Preston for the second time in 3 weeks (this time the signal failed 6 minutes after the previous train passed the errant signal)- last time I physically collapsed at the end of the dash, not quite so bad today (my trip report last time glossed over that, but today was deja vu);
I dodged by good fortune and replanning a suicide on the tracks causing multiple cancellations;
I dodged by equal good fortune and replanning another signal failure which led to a track closure and multiple cancellations;
I had a delay caused by train doors jamming closed;
To maintain connections I have raced across Manchester Piccadilly station twice, each time in 2 minutes flat using real time platform data on my device while on final approach;
I am currently sat with a 1 hour delay due to an absentee train driver.
Current delay total of 98 minutes, could easily have been twice that or more.
As I say a really bad day, but all of those are single events which can happen on any transit system, and you need to allow for.
You have paid a lot for your tour, and had much anticipation. Don't mess it up with over tight scheduling.

Oh, and you are welcome.