Thats one way.
Another way to tell is the UK imports don't have an alarm as standard, so if you've got an extra plip key fob as well as yr standard key, then it could be an import.
Hence, if it's not got an alarm, its an import.
Dont worry too much if it is an import, if its a nice car and youre happy with the price, go for it. Trade-Sales.co.uk is a massive new and pre-reg car seller at reasonable prices, their cars are UK imports - I know, mine was originally sourced from there.
Happy shopping mate.