Most better quality Raleigh bikes from the 70s did not use the 26 tpi threading on the steerer. Best way to check is with a thread gauge and they are pretty cheap and available from places like Amazon. Or, you can compare the threads on your steerer to something with known threads. Also, if the headset that used to be on the bike was not one of those proprietary Raleigh headsets, then it is very unlikely that the steerer is 26 tpi.
Once you figure out the threading, the only other question you are likely to need to answer is the id of the fork crown race you need. Take off the old crown race and measure. It will probably be 26.4 mm, or very close. Some Raleigh forks had 27.0 mm crown races but I think that almost all of those correspond to the 26 tpi Raleigh proprietary headsets.
According to the Sheldon Brown site, NJS headtube diameter is 30.0 mm and so is "proprietary" Raleigh. ISO and any other, just about, British, French, or Italian bike will be 30.2. In my limited experience with 70s Raleighs, included Super Course bikes that came with the 26 tpi steerer, is that ISO (30.2 mm) headsets work just fine (with a proper 24tpi steerer, of course). People with more experience have told me the same. I've never encountered anybody who said that a Raleigh fitted with the proprietary headset had to have the headtubee milled to accept 30.2 mm cups.
The NJS headset that you refer to looks like a nice product and it has the style of the old Campagnolo. Measure the ID of your headtube and if it's 30.0 mm, the cups might not be too loose. But, I'll bet an "ISO" headset with 30.2 mm cups will press right in. Tange Levin ISO headsets with either 26.4 mm or 27.0 mm crown races are about $20 to $25, widely available, sturdy and attractive, though not quite as stylish as the NJS versions. Be aware that the Tange Levin CDS ISO headsets have a quite short stack (33.3 mm) which means you will probably need to add some spacers at the top, unless you want to cut your fork. Advice: don't cut your steerer to match a headset with a real short stack as you might want to use a different headset later.