Most supermarkets have access to Shepherd Neame beers; Bishop's Finger, Spitfire etc, and these sell for a high price; 6-10zl, compared with 3-5zl for local beers. However, you won't find the types of beers found in UK pubs; big-name ales like Greene Man IPA don't have a market over here, nor do the non-beer drinks like cider (strongbow, magners etc).
Ignoring Guinness (not a UK beer), pubs don't stock any UK beers on tap unless the pubs themselves specifically cater to the ex-pat crowd. Even there, ciders and beers will retail for between 12-20zl, compared with 5-10zl for domestic of regional (czech) lagers.
Also, you don't see the foreign brands common to UK pubs. While the big names of Heineken and Carlsberg are available in Poland, foreign brands popular in UK pubs such as Kronenberg 1664, Stella Artois and Fosters are nowhere to be seen in Poland... fortunately.
Bigger drinkers will give better info; I'm not one, but that's something to start with.