what year are your Londoners? I ask because rogers used Zolatone, an auto trunk paint, on the interiors in the 70's, maybe even 60's. It is grey with white speck. Zolatone still makes auto trunk paint in many speckled colors, and their grey\white is dead on match to the 70" drums, I know because I used it to paint the trunk of my 1965 Chevelle, and later used the same paint to do a drum interior. I have seen some earlier "grey speckled" rogers interiors with a little more or sometimes a little less white speck. That is a function of air pressure in your spray gun. The white specks are actually little capsules of white paint that float in the water based grey, and they break open under pressure going thru the gun nozzle, so pressure also dictates how much white speck you get.
But, Zolatone is expensive and not easy to come by and you need a spray gun. I would say the Plasticote grey-white looks real close to the interiors I've seen.
Tell us how it works out.