Will an iPad 2 3G from UK work on a Poland's (or another country's) 3G network? I have the possibility to buy an iPad 2 3G which is written iPad "UK", on the vendor's site. I expect it to have a different charger plug, but that's all.

Being in Poland, will I be able to make it work with 3G cards from Poland?

The broader question is to know if there is a country limitation, or a difference of norms in the 3G matter for the iPad 2.
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