Re: Ummm...
The spectrum is divided into small frequency bands, and the operators buy a set of them.
Obviously you can't have two neighbouring cells using the same frequency, or you'll knacker the speed - which is why they all need several bands to be running at once.
There are also different uses for different bands. The higher the wavelength, the more data throughput you can get, but the shorter the range. So you obviously want the higher freqency bands in cities - where each cell will be physically smaller (to accommodate the extra users).
I also seem to recall the higher frequencies are better at penetrating buildings. But if you'd only got those, they'd be useless for rural coverage, as you'd need many more masts.
As I recall 3 didn't have enough of the 3 and 5 GHz spectrum to be able to run a workable 4G service - and so EE were forced to sell them some of theirs - before getting the licence. But they did the dirty, by selling it, and delaying hand-over for a year, so they got a head-start on 4G sales.