I have sorted possible solutions from best to worst.
Use a GBP account in Polish bank and debit card
In several Polish banks you will be able to open a GBP account and a debit card will be issued for you. At T-mobile Usługi Bankowe, the yearly cost of such an account & debit card is 10 PLN (about GBP 2), so it is close to nothing. When transferring PLNs to GBP account, you should consider using an online currency exchange platform like Walutomat. The exchange rates are much better (at T-mobile: 5.02 PLN/GBP, at Walutomat: 4.79PLN/GBP) and the transfers are almost instant. There is a commission on Walutomat's services, about 0.2% AFAIR.
Note: I am not affiliated with Walutomat or T-mobile Usługi Bankowe, in any other way than being their happy customer :)
Note 2: You need to check at the T-mobile Usługi Bankowe if you can open only the GBP account, or do you need a standard PLN account too. You might be charged for the PLN account if you don't use it, so please check against the banking fees table.
You can try an ATM with dynamic currency conversion
Some ATMs will offer you an exchange rate in your card's currency (PLN). The ATM will display you the exchange rate and the exact amount of PLNs that will be deducted from your account (excluding ATM withdrawal fees, if applicable). If your exchange rate is REALLY bad, this may be a good option. Although the exchange rates at the ATM had been really bad for several years (comparing to my bank), I have found out recently that they are slightly better now. You should be aware of the market/real exchange rate before using this option, however, it may be good for you.
Pay for your purchases by card and use the dynamic currency conversion
Some terminals in shops will also have the possibility to charge you in PLN, however, not all of them, and with every purchase you will need to check the currency rates. It is not comfortable at all, unless you plan to make a one large purchase with your PLNs.
If any of these solutions does not suit you, you can always bring cash to the UK. Naturally, this is not the best solution (that is why it is the last one in my answer), and you would have to predict how much money you would need. The exchange rates in Polish exchange booths (except for airports and tourist spots) are acceptable and should be better than your bank's.
The currency exchange booths do not have card terminals
I do not believe or recall a currency exchange anywhere in Europe that is equipped with a credit/debit card terminal, so that you can be charged in your local currency (PLN), and be given cash in the exchange's local currency (GBP). Even if that was possible, the card processing cost (which may vary 1-5%) would be transferred to you (by giving you a bad exchange rate).