Can you work in Britain with just an identity card? SGM have a Polish guy that has just an ID card and saying that is all he has, can he work in Britain?
EU citizens can work and reside for up to three months anywhere in the EU without requiring anything else than a valid identity card or passport. That right is enshrined in EU law (see article 6). British, Danish and Irish citizens need a passport because those countries have no national ID card scheme but they certainly recognise other member states' ID cards.
After three months, EU member states can require citizens from other member states to register but that's not mandatory in the UK. It is however possible to apply for an optional “registration certificate” and you will note that, in line with EU rules, the form does explicitly mention identity cards as a valid form of ID, as do many other official pages (e.g. about applications for a driving license or National Insurance number).
In practice, I haven't worked in the UK but I have travelled there and worked in Ireland (another country with no ID card, although they will introduce a “passport card” very soon) and I have never had any problem using my ID card for all official purposes. The only small issue I ever faced anywhere in the EU was with a mobile phone vendor in the Netherlands, apparently their system did not support ID card numbers and they insisted on seeing a passport. I simply got a plan from another operator.
All this also applies to non-EU EEA countries and Switzerland but not yet to Croatian citizens (they can certainly travel with an ID card but need a permit to work in the UK).
Employers need to exercise caution here. For example German residence permit cards and Italian identity cards are issued to residents of those countries but they don't prove citizenship, in fact in the former case quite the opposite.