If you have the abilty the best exchange rates can be obtained from Kantors (exchange shops in Poland) I was quoted 4.28 from my FX company but managed to get 4.21 PLN - 1.00 GBP over the counter, only problem is you have to carry cash around with you.
Today I needed to move £2000 from the UK to Poland. The GBP PLN rate on yahoo is 4.1006 right now (when I checked) which I used as my reference.
no1currency.co.uk website rate says 3.813 PostOffice.co.uk website says 3.7482 My bank (Cater Allen Private Bank) quoted me 3.9974 CurrencySolutions.co.uk quoted me 4.0736
The range between the worst (post office) and best (currency solutions) relates to a saving of 600 zloty. So thanks to ct11 and inkrakow for recommending CurrencySolutions.co.uk.