Cultivated Blackberries and "Brambles" are different. The latter can be a bit of a let-down flavour-wise. They're not the same cultivar, though the hedgerow ones can be garden escapes. I don't know if there's a time when the commercial ones are very cheap, but it might be a good plan to buy some up then.
I got to find out about fungi at one time. There's an area near me which is often full of east Europeans, who seem to think you can go anywhere and strip the lot. In this particular case it's illegal. I've also seen some of their confiscated harvest, and looked at it carefully with others. A high percentage wasn't what they thought and was more or less poisonous. With some you need a high powered microscope and spore-print There are one or two easy to identify, reliable species, like ceps, but if you find some only take a few!
Pick-your-own strawberries are a good compromise, because a lot of the commercial ones are flavourless imports selected for size. Some roadside stalls are good . If they have loads, they'll do a deal. They don't freeze well but make good jam.