You can only get it from fox farms for fur and the only exporter is the US I believe and it needs specific licenses to import or something similar.
I can literally go on amazon and am offered various brands all claiming to be Red Fox Urine – apparently, hunters in the US put it on so deer don’t smell them (sounds lovely)! If it should have a license for import I’m pretty confident that a single bottle costing <£15 stands a good chance of getting through customs! I can’t imagine you need loads to be detected by highly trained dogs.
You’ve stimulated me to do a bit of digging and it looks like there’s more than one legitimate drag hunt, but there is one legit drag hunt “representative body” who have been doing it since before the ban (in fact for well over 100 years) and are keen to ensure their drag hunt name isn’t brought into disrepute. The drag hunt differs in that the drag is continuous whereas the trail hunt breaks the trail to make the dogs go looking for the trail again (like chasing a fox). I can see the logic, but that does make it more likely you find real trails even if you were doing it legitimately.
Interestingly the reason the trail hunters claim not to have switched their scents is they think the Hunting Act will be repealed and then they want to switch the dogs back to live quarry. Given the time to repeal an act of parliament, which has not been included in the queens speech or even a party manifesto that seems like a crazy plan!