Its proberly going to come down to money as usual.
All the 121.5 transmiters / DF receivers upgraded to work on 2 freq's, a extra set of boxes in D&D all the cabling "stuff" to pipe the extra about the country.
The Mil got there system years ago when the cold war was on and money wasn't as much of an issue as it is today. I wonder if they decided today that they required a freq networked for the whole of the UK if they would get it.