Does anyone know what sort of rates are charged for this [on average] and who exactly is charging them?
It really depends on so many factors, but at least with ING in Poland, there's three options - either you can pass all the charges to the receiver, you can split the charges (as in I pay for sending it, and you pay for receiving it), or the sender can pay all the charges.
You can also use Transfer wise, which sets the fees out clearly - I'd recommend using it if you want to have certainty over what it will cost and how much will arrive in your account. Generally speaking though, if you want to do a bank transfer, then I'd recommend the person in Poland converts it to GBP, deposits it in their bank, then transfers it with a GBP-GBP transaction instead, which should only have a small fee attached to it.