HD channels over the "air" currently .... None.
There is not enough bandwidth available to broadcast HD channels while the old analog signal is being transmitted.
There will probably be HD channels available by freeview sometime after 2012 when the analog signals are turned of.
And pretty much any TV you buy these days will have a built in freeview tuner to enable you to watch all the freeview channels (depending on coverage in your aera)
more info (coverage and channels etc ) here
http://www.freeview.co.uk/