That's a good point.
I reckon if one of the criteria was "members who attended the Real Madrid 4-0 home game" this one would sell out in no time. We'll have a lot of local families with membership and then a lot of what you would call tourists who want the instagram experience. They won't have been to an FA Cup tie at home to Salford, but they'll probably have paid to go hospitality against the dippers or to a CL game.
It's those fans that would buy the higher cost tickets, there are enough of them out there. But they wouldn't have qualified for a semi final ticket and as I said, the tout sites will be offering tickets for a similar price to the ones remaining. I'm still confident it will sell out though. We'll have people in and around London who can buy tickets next week without the travel costs/issues.