Who decides what temperature trains should be heated to? On the old rolling stock it is possible to open the window when a carriage is inappropriately heated, not so the new rolling stock. A central heating survey suggests that on average we heat our home to between 20-21C, so why do trains need to be hotter? Many passengers are wearing outdoor clothing so surely heating the trains to 23-24C is simply wrong as well as wasteful.
More basically, are the thermostat sensors near doors which open at each stop?
If they are located near doors, they will detect a rush of cold air at every stop and try to counteract that.... It is what the system is programmed to do! I seem to remember stories of both French and German railway companies having similar problems. At home your central heating thermostats are located well away from doors for that reason.
Be thankful that there is heating. Have a look at the equivalent forum for some other operators outside the South East. ( Giving their names would probably get me banned, even though they are not First Group.)
Agree with all your comments apart from the last one. We are all wearing winter clothes and so should be fine even without heating. Issue is when there is heating everyone is wearing jackets and coats and it's 25deg on the train which is much worse. Whoever decided this really needs to have a rethink of what customers really want. Spending this much money on trains just to make the journey less comfortable than the old trains with windows, what's the point??