In short, there isn't going to be one. Not one worth having, anyway. Some time ago I signed a Number 10 ePetition asking the Government to ensure Ofcom reserves sufficient spectrum for a viable HD service following analogue switch-off rather than simply hawking it off to the highest bidder.
I got this response today:
http://www.pm.gov.uk/output/Page14375.asp
In short, no thanks, we think an auction is the best way of dealing with spectrum. Honest guv, we're not simply slobbering over the thought of all the dosh we're going to make.
They are bleating about reshuffling the existing DTT multiplexes to make room for up to four (count 'em!) HD channels but even so, this means that in the long term, the only way to access a decent range of HD content is going to be via cable or satellite.
This decision has basically ensured that the integrated digital tuners included in so many of the HDTVs now on sale, are already staring obsolescence in the face.