Why TNG airdates in UK are 2-4 years after USA initial airdates?

What is the reason, that most (all?) Star Trek The Next Generation episodes have two to four years difference in initial airdate between USA and UK?

I'm from Poland. When TNG started airing in USA, my country was still under communistic rules. Even though, everything "from America was bad" these days in my country, I remember, that we have TNG air dates far more close to those from USA. We watched these episodes in the same year or maybe with maximum one year difference (I think, that TNG started in 1988 in Poland).

This makes me even more curious, why there is that large (?) gap between UK and USA?