Freeview HD won't happen until after the analogue TV shutdown, because HD would not be compatible with the SD boxes already out there, so presumably HD will run alongside SD for a while (unless they decide to "pull the plug" on SD boxes, which is very unlikely). Until the frequencies used by analogue TV are released, there's no room for HD Freeview, and the shutdown won't be complete until 2012.
The London trial will probably be a limited one using only one mux, and it's only possible in London because of the geography and population density.