It was originally planned that all transmitters would have 6 muxes. Ofcom’s mistake was not to mandate all channels being available everywhere. So the commercial companies refused on the grounds of cost versus return on advertising.I believe that the limit on these transmitters isn't necessarily available spectrum, but also cost. They carry only the services that Ofcom required to be made available to any location that previously received analogue service. Regarding spectrum some of these transmitters cover only a really small area, and are already distinguished from full service transmission by polarisation. I find it hard to believe none of them could find more than three available bands at their specific location.j
We're on a Freeview "Light" service ourselves, but don't really feel we're missing anything much.
Since then 20 TV channel allocations have been stolen (sold to) the mobile industry without compensation to the TV industry so it would now be difficult to have 6 muxes at the relay transmitters.