The Thames Diamond Jubilee...

Whats stopping boats that have'nt been registered for the event simply turning up and mooring where they can?

This will probably be the most heavily policed Thames event of all time so the chances of casually mooring up for a view is going to be nil. I believe during the Olympics Teddington Lock is not allowing boats without proof of a reserved mooring to pass through to the tidal section unless they are going beyond the VTS controlled area. Probably the same will happen for the Jubilee sail past.
 
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