Channel 5 is available!
What you will not find are the related Five US and Fiver. The reason for this is because the other 'five' channels are not 'free to air'. They are, in fact, 'free to view'.
The primary difference is, while you do not need to subscribe to a service to receive these additional channels, you do need to purchase a card which you insert into the Digibox in order to receive these channels; hence they are not available to everybody.
Also - technically - because the channels are 'encoded' and require a card in order to view, this means that the channels will not, currently, be watchable on a receiver that is something other than a Digibox.
Sorry...
[however, we do have five now, maybe the others will come in time?]
In early 2005, ITV3 was launched and it was fully free to air (FTA). In November 2005, ITV1 and ITV2 also became FTA. If you know where to look, the ITV1 regions may be selected individually, if you don't like the default Central West. A week later, the new channel ITV4 was launched on satellite, also FTA.
In August 2006 one of Channel Four's separate ventures - Film4 went free-to-air.
The change to them? They now insert advertising into the movies. Some ex-subscribers don't like this, but it is tolerable. Certainly, Film4 show some impressive movies. From their point of view, they've gone from around 300,000 subscribers to a potential audience of millions. Film4 is available on digital terrestrial, satellite, and (I think) some cable services. Lots of great movies, lots of original movies, all free.
The story doesn't end there. I did a scan on 2008/05/05 and came across what appeared to be Channel 4 without sound. The very next day, Ewen SMS'd me to tell me that the four channels were FTA as part of the BBC's FreeSat service. Consequently I added the channels and then watched a double-episode of Friends. I know, I know. Friends is kinda cheesy, but after a smidgen over half a decade without, it was great to see the programme and how little had changed. Kinda like an old friend, you know? ☺
The Autumn of 2008 brought with it various tests of five, albeit without a proper channel ID. I think it was "6553" or something like that.
It wasn't until the winter of 2008 that five came on-line for real. Around the same time, we also had the Channel 4 regions (not that any of them say which region they are!) plus, usefully, the Channel 4+1 versions.