Prefer the M324 and no probs on flat side with Mtb shoes - had the single sided touring pedals (looked like the photo posted by smallfurry) but can be a bitch at traffic lights if the pedal spins wrong side side up when trying to clip in as no solid contact area to push down on ( however I guess your tour route will probably not take you through central London so probably less of an issue!)
Prefer the M324 and no probs on flat side with Mtb shoes - had the single sided touring pedals (looked like the photo posted by smallfurry) but can be a bitch at traffic lights if the pedal spins wrong side side up when trying to clip in as no solid contact area to push down on ( however I guess your tour route will probably not take you through central London so probably less of an issue!)