For the UK & Ireland, an EU identity card is a valid travel document.

For Cyprus, Romania and Bulgaria - the other EU countries which are not (yet) in the Schengen Zone - I can not say.

However, as I recall, as part of the KLM online check-in procedure, you will be asked for your travel document details as part of the UK's API (Advance Passenger Information) requirements. This just means that the UK is one of a growing number of countries that require the airline to collect basic data, such as name, nationality, ID number, expiry date, etc etc on all intending passengers, and pass it on to the UK Border Agency before the plane sets off, for security purposes. As I have only a passport (Ireland doesn't issue national ID cards) and therefore have no experience of completing the required API information using an identity card, I assume that it is a straightforward matter and that you will be able to fill in the details of your ID card rather than a passport (although not having an expiry date sounds unusual).

I would plan on having the passport to hand when you do your online check-in, just in case. But if you check in at the airport, the check-in agent will complete the relevant details for you.


Advance Passenger information advice on the KLM website:
In English