Article 5 of DIRECTIVE 2004/38/EC
provides that
“Without prejudice to the provisions on travel documents applicable to national border controls,Member States shall grant Union citizens leave to enter their territory with [i:800f1b8dcc]a valid identity card or passport[/i:800f1b8dcc] and shall grant family members who are not nationals of a Member State leave to enter their territory with a valid passport.”
Home Office advice to EEA nationals who wish to enter the UK is as follows:
“You need to show your passport or national identity card when you enter the United Kingdom. When you arrive at major ports and airports you should use the separate channel marked ‘EEA/EU’ where it is available. Immigration officers will check your passport or national identity card to make sure that it is valid and belongs to you.”
see http://www.ind.homeoffice.gov.uk/applying/eeaeunationals
So looks like an identity card is fine for entry and as evidence of Polish nationality