TMA QNH already exists, it's en-route QNH you need to worry about.
Anyway 18.000 ft sounds a bit high to me. It's OK in the USA where you have those things called mountains, but I would reckon either 9 or 10,000 (as there presently seem to be several 'divisions', such as floor of 8.33khz frequencies and top of LARS, at FL095 at present) or 6,000ft as it already is in some TMA.