DebConf Organizers blog
If you are coming to DebConf from an EU member country, Iceland, Norway, Liechtenstein, or Switzerland, you are entitled to free healthcare in the UK (not including elective treatment or non-urgent pre-existing conditions).
To prove that you are entitled to this, you need a free European Health Insurance Card. The arrangements for getting one will depend on your country, but if you don’t yet have one you should apply as soon as possible, to get one in time for DebConf.
Residents and citizens of many other countries are entitled to a more limited level of free healthcare, as listed on the DebConf7 website.