Hi there yes Poland is a great option for something cheap, and there are direct flights from many cities in the UK.
I would heartily recommend Krakow as it's one of Poland's best cities for everything, literally everyone I know who's ever been there has absolutely loved it. The city is beautiful, the nightlife is vibrant, and it's not very expensive either, with £ standing at over 5 zloty at the moment! I've been to Krakow myself many times, and will go again.
Go on Google and search under "hen night in Krakow" and you'll get quite a few results on things you can do.
Another very well known city in Poland is Gdansk, you can do the same search for it on Google, although I haven't been there in ages so can't recomment as much as Krakow from personal experience. But the city is very beautiful, and sometimes called Amsterdam of the East, because of the water.
I wouldn't recomment Warsaw so much, it's just a big ugly capital city and you can get easily lost. Krakow is so much better.
Good luck!