Those dodgy EU ID cards such as the Italian and Greek ones are far too easy to forge. The Greek ID card is basically a laminated piece of paper!
They should force them to upgrade to biometric, or make them bring their passports!
I have some Greek friends who just fly with their passport to the UK in order to use the E-Gates.
Mind you last time I used the E-Gates in LHR T5 it said 'seek assistance!'.
Maybe I put it in the e-reader too quickly. Luckily there is always a border force member to help you out and deal with it.