All broadcast TV here in the U.K. has been exclusively UHF for over 20 years, a situation which arose in part because for a long time we had two different standards in use.
Our old VHF system was the original pre-war, 405-line monochrome standard, with positive video modulation and A.M. sound.
There were only two organizations involved, the BBC and the ITA, but co-sited transmitters were very much the exception rather than rule, not least because of practical propagation issues. Most of the BBC transmitters were band I (ch. 1 - 5, 41 to 68MHz) while
all ITA transmitters were band III (ch. 6 - 13, 174 to 216MHz).
In the early 1960s though, it was decided that Britain should eventually move to the emerging European 625-line standard, and a complete UHF network was designed to cover the country, with transmitter sites shared by the BBC and ITA throughout.
Four channels were allocated to each area, two intended for BBC use and two for ITA. The allocations were made so that in most places the four channels were all grouped near to each other, allowing narrow-band Yagi arrays trimmed to the appropriate group to be used for reception. The co-siting meant that viewers would need only one fixed antenna of the correct group.
The first UHF station (using 625-lines with negative video modulation and F.M. sound) opened for the new BBC2 service in 1964, with color (using the PAL system) being added in 1967. Around 1968/69 the BBC1 and ITA services also got UHF transmitters for the first time and went to color.
The three-station network on UHF was gradually extended across most of the country during the 1970s, but the old BBC1 and ITA VHF transmitters continued to simulcast the same program to the old 405-line monochrome standard.
The fourth network rolled out across the UHF system in 1982, and then finally in 1985 with very little ceremony the entire VHF network was closed down.
A lot of good planning went into the UHF network (the co-siting, grouped channels, etc.), but even in a country as small as the U.K. we had problems. Just look at a transmitter map of the country today and you'll see dozens of low-power relays (transposers) in the mountainous areas of Wales and Scotland. In fact when the VHF transmitters were switched off there were complaints from a lot of small, remote places in those areas because they simply could not receive decent UHF signals.
Even over this side of the country where the terrain is relatively flat there are shadows in UHF coverage. There's a town a few miles from here which lies in a slight dip, and even though it's only about 15 miles from a main UHF transmitter with 250kW ERP, they had to build a small relay for the town.
To get to more recent times though, we've seen the rollout of digital TV quite rapidly here in the last few years. Each transmitter site is broadcasting six digital multiplexes, each carrying an assortment of TV and radio stations. You can see the list here:
https://www.dtg.org.uk/retailer/dtt_channels.html At most sites they've tried to allocate channels within the existing antenna group of analog channels.
In many areas though, the digital transmitters will not be brought up to full power until analog is shut down, which is due to take place region-by-region from the end of this year through to 2012.
My local transmitter, Tacolneston, is running two multiplexes at 10kW and the other four at only 5kW:
https://www.dtg.org.uk/retailer/tx_ea.html A lot of people haven't realized this, and on the fringes of the service area (I'm about 25 miles away) some multiplexes can be very borderline at the moment, unless you want to spend a lot of money on a high-gain antenna.
?