It could be done, but you have to have the (non)firearm brought in as if it was live, and to a person/business liscenced to hold that class of a gun. So if it is a ungrandfathered prohib, it limits who it can go to. It then gets submitted for it's deactivation status.
There have been no imports that I have known of. It is possible, but not really economical. With the number of prohibs still in Canada, coupled with a declining number of people liscenced to hold them, and the fact that they can't get ATTs to shoot them, the prohibs are not overly priced in my opinion. I've seen some pretty select guns sit on shelves because the F/A guys just aren't buying like they used to.
Collector's source brings in some dewats from time to time, but it seems like they come more from the Fins, than England. You can always check with them.