In terms of the channels it carries, the number of "timeshares" in operation on these channels, and the regional variations between nations, multiplex A is the most complicated of all the muxes currently being broadcast in the UK. Here's a short guide:
First, a little bit of history. When the plans were being drawn up for digital television in the UK, it was decided to "gift" capacity to the analogue public service broadcasters (PSBs). The BBC were given a multiplex to themselves, ITV and Channel 4 were given one to share, but Channel 5, S4C and digital-only TeleG were given the rights to be carried on the commercially-owned multiplex "A". In the end, a consortium made up of S4C, Five's then-owners UBM, and NTL (now Arqiva) won the bidding process to operate the mux (under the name of S4C Digital Networks - SDN). ITV bought the company in 2005 for £134million (S4C have no involvement anymore).
Second, the channels. Five have been gifted the rights to 50% of the multiplex nationwide. Their half - which carries he bulk of Top Up TV's services and Setanta - carries the same channels nationwide; there are no regional variations.¹
S4C have also been gifted the rights to 50% of the multiplex, but only in Wales. They are entitled to broadcast their own channels in this capacity. The equivalent capacity in England, Scotland and Northern Ireland belongs to SDN, who lease this out to other broadcasters. Only one of these streams, Bid TV, is broadcast nationwide.
Neither CITV, Smile TV 2 nor NetPlay/Super Casino broadcast in Wales. After a great deal of jiggery-pokery, the capacity to carry the former has now become available, but there is no indication that the kids channel will be available any time soon. The absence of NetPlay and Smile TV 2 is because this space is used to carry S4C and its text and audio services. S4C Two timeshares with QVC.
SuperCasino is also absent in Northern Ireland. One reason for this is that gambling channels are illegal in the province; another could be that the space is being kept vacant for Gaelic channel TG4, which will be coming to Freeview when the region switches off analogue signals.
There are ten streams in total carried in England and Scotland. The only difference between the two nations' services is TeleG, which only broadcasts in Scotland for an hour a day (it's own gifted capacity), and timeshares with CITV/CNN et al. This Gaelic channel is expected to cease broadcasting come switchover, when BBC Alba will most likely become available (subject to approval from the BBC Trust).
Many thanks to Ray Cathode, who helped explain all this to me... twice.
¹Apart from the regional variations in advertising carried on Five.
