Oh yes, the later Mk1 seats are excellent. Just compare a 1st-gen DMU with a Pacer or even a Sprinter - no contest. I always say a 1st-gen has three levels of suspension - primary, secondary, and seats - so you can actually sit on one for a while without getting cramped. The Pacer only has one!
It's much the same in Finland. On the long-distance trains there are three distinct types of train in service, including our own version of the Pendolino. The old "blue coaches" are roughly equivalent in age and technology to late Mk1 stock, and apart from the lack of air conditioning and other modern luxuries, are just as comfortable as anything built later. With that said, our Pendolino is hardly bad - the seats recline, there is enough legroom, and you can really relax in them. (The very large Finnish loading gauge helps there.)
FWIW, how many passenger trains in Britain are load 14 these days? I doubt even the railtours usually put that many on. At the right time of day you can see them regularly in Finland - 12 blue coaches (mostly sleeper) and two car-carrier wagons. This despite Finland's low population density.
As for commuter trains, the new-style seats we have are, shall we say, "adequate" for a short journey. The same seats are used for the Dm12 "bubble car" used on low-traffic branch lines, some of which are remarkably long - and the Dm12 doesn't have air conditioning either. But there is still one EMU set and a rake or two of "red coaches" with the old-style bench seating, which is far superior in comfort but, alas, less vandal resistant. The red coaches are brought out twice a day for the longer-distance capital commuters to use, but otherwise stay locked up out of the way - there's only one or two trains that require more capacity than three EMU sets in multiple can comfortably provide.