This might be slightly off topic, but its been bugging me for a while, so I thought I'd ask the question here and see if I get an answer.
I few months ago I was in our local Tesco store and decided I needed fuel. So I rode from the store entrance to the petrol station without my helmet on. (Right or wrong I don't really care, cos I have to take it off to pay in the petrol station anyway

). As I stood at the pump, an off duty police officer approched, showed me his badge and said 'You've just been witnessed riding without a helmet on....' told me that he could book me but in this instance would let me off with a slapped wrist.
Now the question is, this is a private carpark owned by the store (I checked with the store manager the next day), but obviously the public have access because its a car park designed for free public use. Do the same laws apply as on the public highway (which is what the officer said) or, as I though was the case, since its private land the rules do not apply and he was just trying it on.
I don't want this to turn into a discussion of the rights and wrongs of helmet law, I just want an answer as to what is considered a 'public highway'.