The best way for a Polish person to learn English?

G&S on the legitimate London stage is an excellent way to hear the highest of RP known to the English-speaking world:-)

Yes, because it's annunciated very clearly.

Sadly, it's very very rarely on the London stage nowadays, and for decades has been the territory of scarily bad provincial am dram.

Quite.

You've got me thinking about learning English through classical music. Pop and rock have problems sometimes with poor lyrics. Something well enunciated, clear grammar, accessible to A2/B1. It could work well.

The best way for a Polish person to learn English is to use it every moment you are awake and stop looking back. There is no back.

In Poland, English is like riding a bicycle. It's fun to be able but the opportunities are rare.