U.K. Immigration prefers you carry your passport (and not your ID card) to check at immigration, though you will not need to fill out a landing card upon arrival. EEA citizens can carry only their ID card.
You will need a passport at customs as well as information for your landing card (including length of stay and address you can be located at). This also applies for Commonwealth countries.
The U.K. does not make routine immigration checks on incoming flights from Ireland, the Channel Islands or the Isle of Man. That could create a problem if you need a visa activated for your stay. You can enter from any of these destinations and return to them without a passport.
U.K. border officers scan and cross-reference the information on the passport and landing card to ensure only eligible travelers are entering the U.K.
National passports, EEA travel identity cards and 1951 Convention travel documents for refugees are the only forms of identification accepted by border control.
