How to withdraw money from currency account without having to lose so much to currency conversion?
In answer to the "how I can perform withdrawal with the lower rate (having GBP)?" part of your question, as Joe stated you need to use another bank or currency exchange company to convert the GBP to PLN.  Most of the UK banks charge similar amounts, and it's usually not possible to transfer the GBP to a foreign bank unless you have a GBP account with them. Some currency exchange firms are Transferwise, FairFX, CaxtonFX, a web search will show a fuller range. You could also use Paypal to do the transfer (if you have a paypal account) by transferring the GBP from Barclays to your paypal account and then from there to your PLN account.