I am looking for the same but don't worry if you don't pick one up before the gig I have just been to see them in Munich,Paris,Barcelona and Madrid and got tickets outside on the night at every gig.
Thanks for the encouragement. I think you are right but I want to take my two teenage sons and we miss one ticket, and it is a bit awkward to take both of them to Wembley with no certainty we all get in...
I never bother to get tickets for Wembley shows because if you wait till about an hour or so before stage time you will get in for less than face value. Did it for all the Stones shows there in the 90s.