Jez,
Hmm, I got the figures of of Google! Anyway, looking at MY map (The Sunday Times atlas of the world no less) England + Wales is 50,000 sq nms. So using my assumptions that would be 8000' altitude to guarentee constant coverage everywhere in the FIR - a little lower that 24,000!
What fun
You'll have to help me out here as I'm not sure about this but:-
Over these short distances, are you sure the phase shift would have that screech effect? With a wavelength of about 2.5 meters and assuming something like a 10nm overlap that would be 0.05 degrees (as I make it) out of shift. Would that have any kind of noticable affect?
Then again, maybe that's why East and West of A1 are different frequencies...prehaps?