The Polish accent lacks variety. Poland is more than 2 times bigger than the the UK.
Now this is another arrogant British myth!
Although, in the spirit of the traditional Polish tolerance, I must reluctantly admit that this particular one is somehow firmly rooted in reality. The variety of British accents, or just the English accents, is amazing. I can't always locate the speaker on the map, but the differences are quite easy to hear. And then there's the Eton accent, right? One bloody school...
In Poland less so. But:
1. It was different only decades ago. People relocated a lot in the last 70 years, which resulted in melting regional accents, and then the TV added to it. And there was a cultural pressure too, speaking with accent suggested lack of education.
2. It's the question of exposure. You can hear the difference between a Silesian and a Varsovian, I can, sometimes, recognize people from Chorzów, Ruda Ślaska, Podlesie - if they come from the native families, of course, and speak Silesian.
3. There are more distinct accents in Poland, than you mention. Wrocław (Lwów, that is), Poznań, Warsaw, Kraków, £ódź, lubelskie, Podlasie, Podhale, Silesia, Kaszuby are (were) unmistakable, and there are at least traces of them still in usage. Well, unmistakable... some of the differences not as pronounced as the English wild experimentations, but they were there.
Once I heard a 'street interview' from Toruń, on the radio. They were speaking standard Polish, but the sound of it was an incessant, monotonous rattle, I actually needed a couple of seconds to replaytherecordinginmyheadandparseit. It wasn't a slur, pronunciation quite clear, luckily, but the speed of it and lack of melody...
In general, you are right tho. The English can't speak proper English at all :) Except for the Queen. But she is German, innit. Pedantic.