Arsenic trioxide is heavily regulated in the USA as well, but that doesn't mean there are not ways for the average citizen to obtain it if you put your mind to it.
It would seem to me it would be much easier to find a source within the UK than to try to import a heavily regulated substance into the country.
Limes are still a part of the staining process. It would be interesting to know when arsenic was first used, but it is my understanding that it goes back a ways, giving rise to the early stories that keris blades were poisoned.