Will an iPad 2 3G from UK work on a Poland's (or another country's) 3G network?
It's not locked to any one carrier so you should be able to insert a Micro SIM card for your local carrier and it will work.

The catch is that there are different sets of GSM frequencies in use, so you should verify that your chosen carrier uses frequencies supported by the iPad.  Odds are it will work, but better to be safe.

From the specs page, here is what it supports:


  UMTS/HSDPA/HSUPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800,
  1900 MHz)


The broader situation is that the only problematic (less interoperable) 3G iPad is the Verizon-specific model sold in the USA