In reply to Trythallj:
The EHIC (and presumably the GHIC) only entitle you to the same level of state-provided health care that a national of that country would receive, which is not necessarily the same level as we would get on the NHS. You might have to pay for some of your medical treatment even with an EHIC, and it has always been advisable to take out medical insurance as well. I believe most policies require you to have a card so they only have to pay for any costs which that does not cover.
The GHIC doesn't include Norway, Iceland, Liechtenstein and Switzerland (all non-EU) which are covered by EHIC, although there is scope to add other countries in future. Of course in countries which the EHIC doesn't cover you have always needed full insurance.