Hazels nuts grow everwhere, well everywhere where squirrels haven't
already found them.

The dear old squirrels gather a few nuts and bury them for winter
food. Only trouble is they very often cannot remember where they have
left them, and the the next spring the nuts start growing.

There are a number of different varieties of hazel in the UK (Cosford
Cob, Kentish Cob (Lambert's Filbert). Pearson's Prolific (Nottingham)
that are used for production. The native Corylus avellana is not used
for production, even thou some years a good crop can occur.

But I believe most on sale are imported from the states ???