I seem to be in a minority but I find the new LNER / GWR trains (i.e. class 80x) to be very comfortable, in fact I find them to be the most comfortable trains in the UK.

As LNER has a seat selector which does not require you to buy your ticket from them (or even to have a ticket at all), I have never not been able to get a seat with a window if I wanted one, or sit on the side which is not in the sunlight if I wanted to avoid it.

I would love to sample LNER or GWR first class on the new trains, but the most I will pay is ~1.4x the price of standard. In over 100 journeys I have yet to find tickets at that level. Before covid I paid for first class when prices were reasonable, which was around 30% of the time, but not on any of the newer stock.