The main reason for not having an established unicom
frequency(s), is that relatively, the UK is such a small place.
The next time you fly, tune into a busy frequency at an airport 30 -40nm away, you'll then appreciate the range at which an aircraft can be heard transmitting.
Then consider what it would be like trying to talk to another station, on the ground or in the air, with any number of other aircraft trying to do the same thing.
Even if there were 3 or 4 frequencies allocated for different parts of the UK, you'd still have the same problem.
I can't see an easy solution myself.