Hi Agnieszka. You should have a look at the UK Office of National Statistics website and see if you can find the breakdown of figures by region so you can identify where the most Poles are located. Basically any large urban centre will be likely to have a sizeable Polish community. If there's a Polish community there will be business for a Polish shop but you'll face a lot of competition as there are many Polish shops that have been established for several years.
Bit of a chore to find it, but it might be here somewhere ons.gov.uk/ons/search/index.html?pageSize=50&sortBy=none&sortDirection=none&newquery=polish+immigrants+regional
though probably easier to find in a newspaper report like this one theguardian.com/news/datablog/2011/may/26/foreign-born-uk-population