Depends what boat you're using. Type 2 you cruise the north sea, type 7 you can try slipping out into the Atlantic.
Oh and if you got a type 2 port raiding is a rewarding passtime. you're small enough to get away with it (mostly), at least with the poor defender experience in 1939. It also avoids long searches, which a type 2's fuel tanks don't work with too well.
My favorite raid spot in 1939/40 before the big move was Hartlepool. I went there 5 times in all and every time sank a T2 tanker!
And sometimes there's another ship in the other half of the harbour too, making it a one stop shop for a type 2 patrol. (I usually took 2-3 shots to down the T2, an one or two for the other ship.)