Is it Royal Mail 'tracked'? If so you should have a tracking number and you can check its whereabouts on their website. I've had stuff sent to me through Royal Mail a few times and they used a courier at the Polish end, not Poczta Polska. Generally takes about a week.
Well... the post is always slow. I have sent twice a letter between Germany and Poland, and it took around a month, even though they were sent by priority mail. By the way, according to the tracking services, it took much longer in Germany than in Poland (it waited a few weeks on the Frankfurt airport).
Poczta Polska is not that slow with international shipping, the posts of other countries happen to be slower.
You should have a tracking number for your package, then you can check either on the website of Royal Mail (if it's still in the UK), or here: emonitoring.poczta-polska.pl/?lang=en
(if it's already in Poland) where it actually is.