I decided to begin at the beginning of the war with my current career. I can confirm that in '39 the RN and RAF are not as omnipresent as they are in the later years of the war. In my current patrol it is November '39 and I am patrolling in AN59, which is right off the British coast. I haven't seen any destroyers or aircraft yet, but that has also to do with the weather conditions. The wind has been awful enough to keep the RAF grounded, I suppose.
Overall, in my current career, I have had to dive only twice for an aircraft and the second time I wasn't even spotted. I have seen one taskforce of destroyers which I think was out to hunt me, but I could evade them easily, also thanks to bad weather.
So far I can hunt with impunity!