From experience, I don't discount the foreign interest, but I also don't discount the fact that Shamasters crew have been told what say to anyone that asks. How do most of the 18 yr old kids that SM get to crew know that unless they've all been told it?
When Creation came here and couldn't make a profit because the numbers were so low, where were the Germans then? I know the Germans have Fedcon every year which gets a good quality line up and good attendance, so there's clearly more interest in Trek cons in Germany that in the UK, but I still don't know any experienced Trek con attendees who believe Joiner's sold 10500 tickets. By that reckoning, 3/4s of tickets sold will probably have gone to Germans/mainland Europeans. But then experienced Fedcon attendees should also recognise when they're being ripped off.
A lot of UK fans will have blindly thrown their money at this event, but there still aren't enough of them plus the Germans to make up the figure that Joiner's boasting of. And remember, he's as good as admitted that he can't sell tickets for the Sunday.
I don't think that there is a lack of interest in Trek cons in the UK, sure it might be lower than other shows, I also dont know how crowded the German convention/event calander is compared to the UK. Lets say you can only afford to goto one event a year and there is a convention for this new show you have been watching do you goto that one or yet another Star Trek convention?