I believe the advice is to rev your engine in order to make the pedestrian aware of your impatience.

Honestly! Do you need to ask? You must wait until they have left the zebra crossing before proceding.

The Zebra, Pelican and Puffin Pedestrian Crossings Regulations and General Directions 1997

.Precedence of pedestrians over vehicles at Zebra crossings25.—

(1) Every pedestrian, if he is on the carriageway within the limits of a Zebra crossing, which is not for the time being controlled by a constable in uniform or traffic warden, before any part of a vehicle has entered those limits, shall have precedence within those limits over that vehicle and the driver of the vehicle shall accord such precedence to any such pedestrian.

(2) Where there is a refuge for pedestrians or central reservation on a Zebra crossing, the parts of the crossing situated on each side of the refuge for pedestrians or central reservation shall, for the purposes of this regulation, be treated as separate crossings.

Thanks for that, Doc. The wording is clumsy but clear enough: accord precedence = give way (i.e. stop) and within the limits means while the pedestrian is on the road as opposed to the pavement or island.

I'm sure Unthrottled is right to imply that you should wait another two seconds if you're in a coupe. :)