A forest grow at 50N could produce some nice plants as the temperature and sunlight intensity is higher, and the rainfall is lower (helps with mould), plus plants will start flowering 1-2 weeks earlier than say at 55N which would be suicide to grow in a forest. I would only do it in the south of the UK though and even still it can pose problems.
I have grown some monsters inside a forest, with clay soil, but generally speaking I don't recommend it. Sunlight can be a serious issue, and even an open forest with a clearing might only get 4-5 hours of direct sunlight a day... plus it can be very damp, and the trees/forest will act as a barrier against breezes and airflow which is a bad thing as good airflow and breezes can fend off mould. I would just straight up tell you to find an overgrown field or piece of exposed heathland/countryside, in the middle of nowhere, and plant there where it is out in the open, so it gets good airflow and all day sunlight.
So basically stay out of the woods. If you really have to, just stick 1 or 2 plants in the forest, and have the bulk of your crop in an open, exposed site, say on a south facing hill. You'll then be able to see the differences between both sites...