Well... JAR-OPS says you need a life jacket for any flight farther than 50 NM from shore... I guess the North Sea is a bit bigger than this, so this requirement should apply
[and here goes another one of these new smileys...]
Furthermore the life jacket in question "must be stowed in a position easily accessible from the seat (...) of the person for whose use it is provided". So if your life jacket was in the flight attendant storage, it is not good enough... Hey, maybe she intended to give you personnal swimming lessons just after evacuation...
Cheers,
-- TTF.