The bureaux de change at airports usually have all currencies available (without pre-ordering) for any destination served from that airport. (i.e. you may well be able to get Polish currency at your departure airport).
For greater certainty, why not pre-order you currency for collection at the airport? Travelex are represented at almost every airport in the UK (and probably at all airports with flights to Poland). I've just tried a dummy run on their website and I had no problems pre-ordering Polish currency for collection on Thursday (or any date thereafter) at a random airport (I used Cardiff for my test):
http://tinyurl.com/2epbbc
Chris