I think I can fairly say I've yet to read an account of Enigma/BP/Ultra that doesn't give full credit to the Polish effort prior to the opening of WWII.

The Poles handed all they knew of the German forces version of the Enigma machine-encipherment system to the British. This was not complete information but it was all they knew. They also provided the concept for the bombe which Alan Turing and others went on to build.

The U571 storyline is a compilation of many events from the Battle of the Atlantic which enabled the intelligence service to read Naval enigma. (The Kriegsmarine having developed a particularly complex form of enigma machine). The struggle against the U-Boats ultimately determined when (if) the invasion of Europe could occur.

The U571 movie however, unlike any book I've read, appears to give no credit to either Britain OR Poland for their roles, and claims not only the defeat of Enigma but the provision of Ultra for the USA. If this is true it is playing fast and loose with the truth. (to say the least).

This is the important bit:
Enigma is not Ultra. Enigma was a machine-encipherment system that the Germans believed was unbreakable. Ultra was the intelligence that GC&CS (Bletchley Park) provided to the allied command, using, amongst other sources, enigma decodes.
Welchman's The Hut Six Story is probably the best account of what went into Ultra as he wasn't much involved in the decode process.

AndyG