It would be wise to have all supporting documentation with you to prove the origin of the money.
It would be very wise indeed - basically a letter from the lawyer who completed probate. If this is a cash thing, It should be in high denomination notes, and carried in a wallet, to avoid issues at customs (very rare between PL and UK, but best to be careful). People do carry larger amounts of money, however those who can also carry proof of the money's origin generally have less stress going through the green channel at the airport.
he cannot do a bank transfer
You should ask him why he can't do a bank transfer. If he maintains he can't, ask about it being paid via WorldFirst or Transfer. If he won't do that, you should make sure he gives a clear explanation why. I cannot think of any honest reason that you would need to travel to Poland to collect cash personally. If they need a signature at a notary's office, it should still be possible to transfer the cash in the normal way.
I suppose a lot depends on how well you know and/or trust the relative in question, and whether or not the money is in a bank or for whatever reason in cash.