This question already has an answer here:
I hold Lithuania citizenship and currently live in Denmark. Is it possible to travel to UK from Denmark and back using only the identity card?
According to gov.uk
You can enter the UK with either a valid passport or a national identity card issued by a EEA country. It must be valid for the whole of your stay.
I am fully aware that this question was asked a number of times, for example here. However, since "Brexit" situation might have changed. Moreover, recently while travelling from Copenhagen airport to Dublin I was asked to prove validity of my identity card using additional document (e.g. driving license).