[tackem (OP)]

Can we grow walnut trees and harvest walnuts in the UK?
A healthy superfood growing in everyones back garden/town park? Free pick your own food?
If we cannot grow them in our climate could they be a valuable cash crop in many countries just getting by with subsistence farming?

[alancalverd]

Yes, and they already are a valuable cash crop. Intgroduced by the Romans, they grow fairly well in the UK .  Southern Europe is more reliable for commercial quantities of nuts but the slower growth of British trees produces a tighter-grained wood.     

Walnut trees take a long time to mature and the market for the flesh is pretty well served by existing farmers and processors.

Powdered walnut shells are used in huge and quite cheap quantities, along with high pressure dilute cationic detergent,  for cleaning grease, soot and rubber from runway lights.   I find it amusing that some people are persuaded to buy expensive and abrasive "walnut facial scrub" and "micellar water" to remove the expensive greasy gunge they put on their faces every morning.