In general, Eastern coasts of England and ENglish Channel are covered with very dense minefields... after similiar expierience with hitting a mine, I avoid going close to the shores... north of Irealnd or Western Approaches and slightly west off of Western Approaches are good places to hunt as well and the risk is not that high.
Still, there are many destroyers and planes
In general: deep waters close to the continental shelf are perhaps the best place. Too deep for mines, still quite a lot of targets and in case of ASW it`s deep enough to make few tricks. Shallow waters near England, even in 1939, are risky.