Roger,
I understand from earlier Forum contributions (see below) that all these “radiator taps” are 1/4" BSPP (19 tpi). I assume yours is in the steel pipe at the bottom of the radiator, whereas the XK 120 has them in the bottom of the radiator itself.
The drain tap on the block seemed to have the same tap C.980 but I also see C.2354 mentioned which might have UNF 1/2" x 20 tpi.
If you don’t want to return to the original drain tap in your bottom steel pipe, you might consider to tap the hole slightly larger and fit a UNF 1/2" plug. See more below.
Bob K.