Firstly, there's enough bandwidth for potentially three new multiplexes (in addition to the local ones) to be auctioned off in 2012/13, after switchover. That is, after all, the entire point in switchover. However, the 800MHz band (most of the 60s) will probably be allocated to 4G, which again, I welcome - the three new muxes, if they appear, will be in the 600MHz band (most of the 40s).
A simple trip to Wikipedia will tell you why the whole UHF spectrum can't be used - it's assigned to other things:
380–395 MHz: Terrestrial Trunked Radio (TETRA) service for emergency use
430–440 MHz: Amateur radio (ham - 70 cm band)
457–464 MHz: Scanning telemetry and telecontrol, assigned mostly to the water, gas, and electricity industries
606–614 MHz: Radio microphones and radio-astronomy
470–862 MHz: TV channels 21–69 (channel 36 used for radar, channel 38 used for radio astronomy, channel 69 used for licenced and licence exempt wireless microphones, channels 31-40 and 63-68 to be released and may be made available for other uses by Ofcom. Public consultation due December 2006)
1240–1316 MHz: Amateur radio (ham - 23 cm band)
1880–1900 MHz: DECT Cordless telephone
2310–2450 MHz: Amateur radio (ham - 13 cm band)
In addition, aerials are designed for the 21-69 range of channels, so anything further out than that would require new aerials unless you're right next to the transmitter.