Originally Posted by graraps
It depends on the nationality of the person. The information submitted by posters is true for US/non-EU nationals. I have no problems entering the UK with my Greek identity card (or any other identity document for that matter).
I don't think you'd be able to enter the UK on "any other identity document". EU national identity cards are formally recognised as travel documents for travel within the EU (and to some other countries that formally accept them for the purpose, such as Switzerland). You can't enter the UK on anything that is not formally recognised as a travel document - your student card, staff badge or bus pass won't do.