Note that, despite employing an All-Star Cast of British character actors, the film, against all odds, has no shared cast members with the Harry Potter series. Not unless you count stuntman Kiran Shah.
Gibbs from Pirates of the Carribean is playing the officer that stops them outside of the concert hall.
Money, Dear Boy: Richard E. Grant has cheerfully admitted that money was his primary consideration for appearing in the film. It's hard not to suspect that it was a primary consideration for most of the other participants as well.
So My Kids Can Watch was also a motivator for Grant, as his pre-teen daughter was a big fan of the Spice Girls at the time.