If germany had been more successful in the battle of the atlantic, what would have happened? supply lines to britain would have been disrupted. hitler would not have had to fight a two-front war. the salerno invasion would have been impossible for the allies to attempt. the allies would have failed at the battle of the bulge due to lack of supplies.

The chance of German U-boats dominating the Atlantic was very slim as with destroyers for bases, Convoys, America joining the war and the cracking of the enigma code the Allies became very efficient at hunting down U-boats. 

Supply to Britain would be dramatic as the Britain relied heavily on her colonies and overseas import for food and natural resources. This would of hurt Britain so much. 

Britain wouldn't of been conquered even with the island half starving as getting onto the island was difficult. Britain would still force Hitler into a two front war but due to the success of the U-boats it wouldn't be as strong as the one in our timeline 

Battle of the Bulge might of been lost by the allies, however this wouldn't change much for their situation as they already had plenty of troops on the continent, and the Germans would of run out of fuel

Salerno invasion still would of happened, as supplies could be brought from India, Africa and other British colonies 

To answer your question, Supply lines to Britain would have been disrupted 

The fact that farmers were paid for destroying crops.