St. Nicholas Cathedral Newcastle-upon-Tyne
St Nicholas Cathedral, Newcastle-upon-Tyne was chosen for the initial location of Richard and Donna's wedding.
As you can see from the picture it's also an ideal site for Forrest's attack. West of the Cathedral main door is a building with a flat roof. At the back of this building you can see a wall which could be abseiled down, into the alley there. To the North you can see the car park from which Forrest planned to watch. The streets here are one way and an ambulance could quickly whisk away the victims.
Lady Triplet, of course, knew all this and had no intention of holding the wedding there.