It is also odd that at 27.5W not all the BBC channels are there. Certainly either BBc4/Cbeebies HD is not there. Someone posted that it is because the channel is on a different DTT "MUX" (but I don't really understand what that means) and, in any case, why would it mean that no backup is required?
The ~80 main Freeview DVB-T/T2 transmitters in the UK are fed by fibre.
The many small transmitters around the country receive one of the main transmitters off-air via UHF antennas and rebroadcast Freeview that way.
A MUX (multiplex) can be thought of like a transponder on satellite where one frequency carries a number of digital channels.
The 27.5°W feeds are just a back-up; mainly for small transmitters where the UHF terrestrial feed from their main parent transmitter isn't 100% reliable (signal fading problems etc) and might fail sometimes.
The 27.5°W feed carries the contents of two MUXes; both operated by the BBC. All of the BBC SD TV services and radio services are on a MUX called BBC-A on terrestrial. The 27.5°W feed provides BBC One England (network), Scotland, Wales and Northern Ireland variations which are reassembled as appropriate by any transmitter using the feed. The HD channels on 11495 are a feed for the BBC-B MUX which carries BBC One HD, BBC Two HD, ITV HD, Channel 4 HD, and BBC Three/CBBC HD. The reason BBC Four HD and BBC News HD are not on there is because they are carried on a commercially-operated MUX which does not have satellite back-up.
