What is the thread form on a 1" fork? It should be 1" - 24, regarless of whether Italian or British. Is the form British or ISO, or does that not matter? Given that both British and Italian are 1" - 24, what is the difference between them?

Really, the question is do I need to buy Park's insanely expensive die or will any 1" - 24 ISO die work?

The difference is the thread angle: ISO/English threads have a 60° angle, while Italian threads have a 55° angle. ISO/English and Italian are functionally interchangeable, but mismatch will slightly reform the threads, so it is discouraged to go back and forth between them. Once you've installed a mismatch, stick with it.

N.B. with an aluminum headset on a steel steer tube, it is the headset parts that will be reformed. With a steel headset on a steel steer tube, both the headset and the steer tube will suffer reforming.

Then there is Raleigh and French threading as well.

Yes, Raleigh used Whitworth (55° thread angle) 1" x 26tpi on some models, mostly 3-speeds and some of the low-end bike-boom era 10-speeds. Hozan offers a die for that.

French thread steer tubes follow the 60° metric standard, but are 25mm diameter, not the 1" the OP asked about. VAR offers that die.

There are also 1-1/8" x 26tpi and 1-1/4" x 26tpi threaded steer tubes; again, outside the scope of the OP's question. Hozan can supply those as well.