Hello,
Having set my main account in GBP and been living in the UK, do I have to change the currency to PLN when topping up through the app, to be able to withdraw money during a visit to Poland? Or can I simply withdraw from the amount I have available in GPB? If that’s the case, any additional charges?
During the week it does not matter, unless of course you are interested in a particular exchange rate. During the weekend and holidays it would be advisable to exchange GBP to PLN beforehand, as you’d otherwise pay an additional surcharge. Keep in mind you have a £200 withdrawal limit a month, exceeding it gives you an additional 2% surcharge.
(replying within 1h to OP - my reply will appear delayed owing to review process for new users, so apologies if it appears when the question has already been answered)
You don’t have to top up to PLN but it is a good idea to have some funds in PLN so that you don’t get a worse rate at the weekend.
With Poland there are no issues. The problem is countries like Croatia where you still (!) cannot top up in HRK (or Hungary where you can only do it by glacial pace bank transfer).
Avoid Euronet ATMs, these will either apply a fee or a really bad exchange rate
Ah yes, should have mentioned that Euronet is always to be avoided like the plague. 
And with Premium you can get up to £400 (for UK issued cards).