As far as I know, it's impossible to find a modern headset with 26 tpi threads. You could use a non-Nottingham fork, of course. Or you could find an old Raleigh headset on eBay. (You say you want modern components, though.)

As far as bottom brackets go, Phil still supplies them with Raleigh threading. Or you can use Phil rings with a Shimano UN72.

That said, the Nottingham Raleighs are renowned for their robustness, so a headset and bottom bracket will rarely need replacing.