I know one in Central London (near Oxford street). Prices vary according to length of course and how many classes a week you have. They do classes starting around 6pm and 7pm for 2 hours either once or twice a week. Average price is around £200 for a course and there are different levels, but best check out their website for all the details: polworld.co.uk. I know some others in London too, but this is the cheapest Ive found.
I started a course there a while ago, have to admit I dropped out, but am planning to go back.
I have a Polish background anyway, with a lot of family in Poland. It would be nice to be able to hold a good conversation when I go to see my brother and sister there, and with my Polish friends here in the UK.