I don't recall, either, and even Wikipedia doesn't have much, but apparently, there was http://en.m.wikipedia.org/wiki/British_Military_Mission_to_P.... There also is operation Freston, an SOE operation into Poland in 1944. In neither case, I don't think the Brits fought against Poles, but certainly in the second case (at least, that's what the many links with limited information point at), they invaded Poland. Operation Freston even has a book: http://www.amazon.co.uk/Operation-Freston-British-Military-M...

(Given the scarce information, this could still be a hoax, but if so, I fell for it)