The only real problem I can foresee in the UK are the extra words that will inevitably be bolted into every transmission "ABC623 Descent now altitude wun tree tousand QNH wun zero wun zero" instead of "ABC623 Descent now flight level wun tree zero. " Another four for both parties... unless of course we can get a bit smarter. Does Mode S pass an aircraft's current altimeter subscale setting?
PM