Peter, I take your point about finding a larger venue when a big audience is expected: this is something which sometimes can be difficult for festival organisers to predict. Yes, probably the New Rope String Band could have filled the Ham marquee.
However the Bedford is of adequate size for many of the concerts that take place there, and if I want to go there, I turn up after the queue has gone down and usually get in. I just thought "Dump the Bedford" was a bit strong and would be tantamount to biting the hand that feeds you!
And the way to guarantee entry to a particular concert is to buy a ticket for that specific concert (which, of course, you will not want to do if you already have a season ticket)