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.
Unless you are sitting bolt upright on those seats then the standard class seats for me are very uncomfortable. If you feel from the front of the seat, there’s a hard bar or something like that about a third of the way down, that stretches across the seat under the ‘padding’. As I was not blessed with an ample backside if I am not sitting with my back against the seat it’s uncomfortable. YMMV.