I passed through a British minefield on my pre-war shakedown cruise (almost ending my career before the war had started
).
Regarding density, watching through the external cam, I'd say the mines were placed about 5 boat lengths apart - so every 300 meters or so. Looked dense enough to me, although I did manage to make it out alive.
The location of the British home water minefields (which IIRC, were widely published in order to prevent accidental merchant sinkings) are accurate, as I am sure are all the others. I was surprised to find them in place in August/'39.