Re the election, they will send you a grumpily worded fine notice,
(rather than a please explain letter, etc), but you just need to reply
that you were outside your local government area - IE, in the UK.
They typically do not need evidence, but you could always ask the
Border Agency if they can stamp your passport as you leave the UK; or
hang onto your boarding pass. Shengen member countries automatically
stamp and entry to, and exit from, the area.
You can bring most canned foods, etc back too.
My friend's son ate my can of this:
http://www.heinz.co.uk/ourfood/pasta/mini-meals/creamy-spaghetti-carbonara
Julian VK2YJS.