I’d have to check, but I’m pretty sure that virtually all the UK fuel supplies were imported as refined products from the US. The major consideration was IIRC shipping - refined fuels are easier to handle and require less shipping space, and with the Battle of the Atlantic on this was a very major consideration.
Personally I think the Germans did the best they could in the Battle of the Atlantic given their starting point. The major bottlenecks were in crew training and building the boats, so unless you put Speer in charge of production with a mandate for total war in September 1939 you aren’t going to make any substantial improvements to your Atlantic U-boat force. A force that was eventually crushed by the USN, RN, RCN and RAF.
If you do put Speer in charge of U-boat production with the necessary realignment of priorities, you’re showing a somewhat unrealistic level of foresight - remember at this stage Germany had only managed to take Poland. If you wait until the end of 1940 (the earliest stage at which it might have been apparent they were desperately needed) you aren’t going to do a lot better than actually happened.
