I just used some matt black the other day and it dried fine and with a matt finish. My work space is in the basement and it is always a little cool down there. Can't even say I have had problems with it drying either despite the cooler temps.
Could it be a bum tin?
There are probably many people like you looking for these coaches. The demand outweighs the supply therefor the price goes up. It is the same with the Intercity coaches (swallow/executive & blue&grey)
These may help. With exception of the sleeper I don't think there has been a correct royal coach made.
http://www.railwayscene.co.uk/view.php?cat=Royal%20Train
http://en.wikipedia.org/wiki/British_Royal_Train#Fleet_from_1977
http://www.rmweb.co.uk/community/index.php/topic/20583-royal-train-roof/
Damn !
I must confess though I use a Dynamis and it also allows you to name them, not sure if other systems do. I would then go with 5703, 5708 & 5711.