Obviously there should be a common transition altitude across Britain. I think Rat 5's suggestion makes good sense.
But that's a separate issue from the QNH that you dial up when you are below the transition altitude.
Ledhead, 29.92 inches (1013 hPa) is the setting used everywhere above the transition altitude, not below it.
Mach Jump, I'm surprised you can fly so far in Britain without bumping into controlled airspace somewhere along the way! Does it require lots of twisting and turning?
In Australia you can fly thousands of miles without a clearance, and there are some parts of the country where you can't communicate with a ground station on VHF even if you are at 10,000 feet.
ShyTorque, I interpreted "out of radio contact" as "out of radio range."