Cryptologists can crack virtually any code. We cracked the Japanese code as well as the German code. In fact, I think it's safe to say that there has never been a significant code that hasn't been broken.

As much as I hate to quote from wikipedia:

The exact influence of Ultra on the course of the war is debated; an oft-repeated assessment is that decryption of German ciphers hastened the end of the European war by two years.

Despite Winston Churchill's hyperbole on the matter, virtually no historian would support his assessment. Not only that, but it's arguable whether the UK won the war at all. It was bankrupted by the war, it lost its empire and faded into the shadow as the junior partner in a strategic alliance with its former colonies (the US). And the UK certainly would have lost the war without the US and USSR. It would have been possible for the UK to lose even without Operation Sea Lion, if Admiral Raeder's suggestions to Hitler had been taken each time they were made.

The Polish Enigma codebreakers did not really help the efforts in the convoys, by the way. Naval Enigma used different codes and it wasn't broken until the HMS Bulldog captured U-110. Even then, breaking the Naval Enigma keys was of only marginal assistance. The real help came in the form of radar and the American entry into the Atlantic convoy war prior to our formal declaration of war. When we look at convoys to the Soviets, the Enigma factor is negligible. PQ-17 is sufficient evidence that the Enigma codes did not help much.

You also say that Africa shut off Germany from the rest of the world, particularly its oil supply. This is not true, since 80% of Germany's oil came from Romania. It is true that if Africa had fallen to Rommel, the UK would be cut off from most of its oil, certainly.

Taking the hyperbole of people who were understandably praising the efforts of codebreakers throughout the war at face value, and then using that to say the Polish contribution to the war was "decisive", is a careless disregard for the actual facts of history and little more than a lot of backslapping to ease the perpetually bruised ego of the Poles.