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@Scilly.Suffolk
If you check the last link I posted (Sheldon's threads cribsheet), you'll see that the only difference between the three is that Italian threads are cut at a slightly different angle.
In practice, this means that they are interchangeable.
The only part which isn't is the crown race (the part which fits on the forks). Chances are that your bike has a British/ISO standard headset, in common with the bottom bracket. You can get a pair of digital Vernier calipers off Ebay for less than a tenner to check.
If you buy a headset with the same type of bearing as the one on the bike (ie loose/caged ballbearings, needle roller or cartridge/sealed) and the new race doesn't fit, then you can reuse the old one.
Yes, the BB seems to be ISO: the shell could have worn or your measuring is a bit off, but you can confirm this from the fixed/drive side cup: an Italian cup will screw in normally (ie clockwise); an ISO cup will work in the opposite direction; again, check Sheldon's table for details.
You can buy BB/chainset bundles, but mostly they are sold separately so you will need to check what length spindle a particular chainset needs: it should say in the ad, check the manufacturer's website or ask the retailer.