Compared to Germany, Denmark, Switzerland, Holland. Spain. E.g. Once travelling Copenhagen to London. The train from Copenhagen to the airport is spotless. The train from Stanstead to London is dirty and full of litter. There was a pile of rubbish on that train which was waist height; burger boxes, bits of foods, drinks cartons. I was embarrassed to be British. Also nobody cleans the crevices on British trains there's always black grime in there and the toilets often malfunction and stink out the carriage.

I've been using the trains in Britain for a couple of decades and I've never seen anything that bad. Sounds like you were unlucky.