Heard on Radio4 a week or two back, was a piece about there no longer being reciprocal arrangements (I think it was for Jersey, but might apply also to Guernsey) with the UK.
In effect, if you fall ill and need hospital treatment, then it will not be paid by NHS.
E111 does not apply (but in any case, that is limited to healthcare - if you had a heart attack in France and needed to be flown home, or even for a relative to come to see you in France, would not be covered)...
Therefore travel insurance for medical cover is a very good idea, if not essential!
Just that many consider the Channel Islands as being part of UK, and while there is a lot that may make it appear so, like BBC radio stations for each island, health cover is no longer something you can count on.