You need to be aware that legally transporting across borders currency and/or fiduciary notes is limited by total value (I think around €10,000 per person) and if you were caught with more than what is permitted the consequences might be very severe. Even if found to be carrying an amount within the limits you might be extensively questioned as to the source, etc., etc. I have no knowledge as to how Polish banks might react if presented with large amounts of foreign cash for exchange or which rules and regulations exist there - there may be some form of currency restrictions in force. You can google a large choice of non-mainstream-retail-bank options (within UK) for exchanging and transferring which you may find offer a more attractive rate than you have been looking at. In my opinion, taking large amounts of cash with you is a poor option and I certainly would not advise contemplating putting into checked baggage. I do, however, sympathise if the (UK?) banks are giving themselves fat margins on the transaction you are looking for.