I think you missed the link, but I managed to find
this with a moment's searching. Now, I hadn't seen that article before, and presumed Nuvistors to be purely American. Still, Mullard were certainly in a position to afford a little each-way bet on that horse, even if smaller valve manufacturers didn't dare.
At any rate, the 6V3 / 135 mA heaters would have rendered the Mullard Nuvistors unsuitable for use in British TVs of the day. So were they after the Continental TV market, or laboratory instruments?