Which part of Europe and which part of the UK? The new class 380s and modern 334s in Scotland are perfectly solid trains and easily the match of any international units I have used. Some trains are dated, but the internal and external conditions of most UK units easily match those in Germany, Italy and France which are often laden in graffiti inside and out.
Being Swedish my experience mostly from Scandinavia, and Germany/Benelux. But also other parts of Europe. My trips to UK has mostly been to Scotland, but a few visits to England.
The British loading gauge is a big disadvantage, British trains feel cramped. So far I haven't had the opportunity to try the 380 or 334. But there are nice trains in the UK, I've personally become very fond of Turbostars for shorter journeys.
But there is a lot of older stock. I've travelled on a class 158 to Inverness, and for a 3.5 h journey the comfort wasn't great. And last year in England I had the opportunity to try a Voyager, and it wasn't the best train trip of my life. It could use better seats, and the diesel engines were noisy. And while there are modern trains in the UK, the HST/IC125 is still used on a lot of mainline services. And though it probably was a very modern train in the seventies, today it can't be compared to a German ICE, or the Swedish X2000.
Which leads to "lack of electrification", there are a lot of diesel trains. Most of the ones I have tried (except the IC125) have been a bit noisy. And idling diesel trains at major stations is not the best recipe for a nice environment. My first trip on an IC125 was from Edinburgh to London and when I arrived at the platform at Waverley I found a diesel train idling at a platform. After a few hours journey, with a diesel train on a fully electrified ECML, I arrived at Kings Cross where there were a lot of idling trains at the station and the smell of diesel exhaust was unavoidable.
And something a bit annoying is that the only train companies I've encountered that don't offer "print at home tickets" have been in the UK.
But everything is not bad. British railway stations are in my opinion among the best kept in Europe. The fact that even smaller stations are manned is great! (And there are a lot of very beautiful stations in the UK!) And I have the impression that a lot of things are happening, the electrifications and HS2 are great news for the future! And a lot of operators seem to be ordering new trains, new electric trains and bi-mode trains will probably be great for station environment. I'm really looking forward to trying the new class 800/801/802s!