8 day itinerary

We loved the Brooks Guesthouse in Bath as well as the Scallop Shell Restaurant.

Day 6 you have quite a list for one day. We have done all the places you listed, but in two separate days.
We did Wells and Glastonbury as well as a gorge coming back to Bath. That took up most of a day.
In 2013 we did Salisbury, Stonehenge and Avebury in one day driving from near Heathrow. I suppose that you can do Stonehenge and Avebury in about half a day, but it will not be easy. If you try to do it all in one day you will have one very long day or just won't make it.

As for the Cotswolds, we stayed in the Volunteer Inn at Chipping Campden. Rick Steves guide on Great Britain has great guidance.

Definitely rent a car to explore the Cotswolds, there are lots of beautiful villages to explore.

The train to London leaves from Moreton-in-the-Marsh and takes just over 1.5hrs to Paddington. Avis car hire is based there which might help with your car rental plans: https://www.avis.co.uk/drive-avis/car-hire-locations/europe/united-kingdom/england/moreton-in-the-marsh/moreton-in-the-marsh

Difficult to say what the weather will be like especially given the extreme summer the UK is having right now. If you're lucky you might get the tail end of an Indian Summer but equally it might be cold and rainy so take some warm layers just incase.
Autumn is a wonderful time to visit, make sure you go on some country walks to soak up the scenery, along with some great pub lunches of course!

I think on Days 6 and 7 you're biting off more than you can chew, or at least more than I would try to take in.

If you only want to take a quick glance at each of the 4 destinations listed on Day 6, it is doable, but you will not have time to do any exploring. Stonehenge alone is worth a good 2 hours, from the time you park and go through the (excellent, indoor & outdoor) visitor centre, take the shuttle bus or walk out to the henge and listen to the audio guide, then return and have a browse in the gift shop. Avebury circle is a mile and it's somewhat uneven ground, so to simply walk it without pausing to take photos or anything will run you a good half hour. If you want to walk up Glastonbury Tor, I suppose it could be done in half an hour. Wells ... you get the idea.

Late September weather could be mild and dry, or it could get chilly and rain a lot. Bring clothing that can layer and adapt.

Thank you so much! We decided to add a day to our travel based on your feedback! Very much appreciated.