I've never heard of putting cardboard cutouts in the stands at venues here in the UK, but it sounds like London Bethel might have to resort to that, if the experiences here in the next few weeks are anything like the ones we're seeing in the USA!
The main UK conventions are starting soon - Brighton this weekend (14-16 July) and Glasgow and Milton Keynes next weekend (21-23 July) - so it will be interesting to see the numbers at those venues.
The London conventions will not be until 18-20 and 25-27 August (at Wembley and the Excel Centre), but as they're the last ones in the UK and usually the most attended, the numbers there will be very interesting.