White wall tyre were never fited here in the UK except for push bikes, not sure they would have even been available, so it would be a no no for me just not inkeeping with the car.
I think the white walls were popular in a America due to the size of the side walls, they made the tyre look more low profile,
I have seen old US cars here at shows without white walls and the solid black sidewalls look huge.
The father of a local friend here (Portland, Oregon) was once a British car dealer. Among the marques he sold was Triumph. His dealership building was actually more like a storefront at a strip mall, and with such limited room he parked some of his new car inventory in his driveway at home!
This first photo shows his dealership. This was in Beaverton, Oregon, a suburb of Portland, in the mid-1950s. Note the self-service laundromat next door - the same size as the British Motors car dealership!:
The second and third photos show some of that inventory at his home. These are new small-mouth TR3s:
Note that the center car in that lineup of TR3s - the one with the whitewall tires - has a red hardtop. That hardtop was taken from a red TR3 as a mix and un-match exercise by the dealer. The white car did not come from the distributor with a red hardtop. Here's a rare color photo of that car, although of course the color in these very old prints fades over the decades it is still possible to clearly see that it was a white car now with a red hardtop:
Regarding the whitewall tires, my question would be, When, where and why were the whitewall tires put on that car? Obviously it is not an accident. Someone deliberately replaced the new blackwall tires that the car would have worn when leaving the UK, so perhaps the distributor switched the blackwalls for the whitewalls? Was that just a way to mark up the price, anticipating that the premium for the whitewalls would more than cover the distributor's cost of the swap?
It seems a little odd that they would remove new blackwall tires to make this change, but if the factory didn't do it, and the dealer didn't do it, then it must have been the distributor, and the distributor wouldn't do it if it lost money doing so.
Anyway, thanks for the good leads on finding whitewall tires, and thanks in general to all for the discussion on this subject. I appreciate everyone's interest and contributions.