Having worked in two countries, and flown VFR & IFR in one of them, where the TA is 13000ft and TL is FL150 it seems that a number of the posts are making a bit of fuss out of nothing.
New Zealand and Oman are both about the same size and shape as the UK, so make good examples for comparsion. Both countries have sizeable mountains - in the 12-13000ft range.
Oman has fairly beign weather and very little traffic below the TL, consequently it's three QNH Areas/Zones are sufficient. Below the TL all aircraft were given the appropriate Zone QNH.
New Zealand on the other hand has significant low level activity, more than half the country is deemed mountainous terrain and the weather is not only changeable over a very short distance, but can be extreme. Here is how it works in NZ -
http://www.aip.net.nz/pdf/_aENR_1.7.pdf - Note there are 12 QNH Zones.
Generally the Zone QNH is the reading from the major airport in that Zone, so the majority of IFR taffic have little need to change from airfield to zone and back again.
The major TMA's are contained in one zone - so all aircraft in that TMA will be on the same QNH.
When on a cross country (VFR or IFR) aircraft are provided with the appropriate Zone QNH prior to entry to that Zone. All VFR position reports are acknowledged with an update of the appropriate Zone QNH - this may include adjacent Zones.
Personally, I would think a low TA/TL around 5-7000 ft would be a pain, both for pilots and controllers. For the controller, when working an approach sequence, the loss of one or even two useful levels so close to the approach commencement level, must make for some wasted space. For the pilot, both on approach and departure, having to change the altimeter setting during high workload periods (fast climbing jets, complex STAR procedurs) must be distracting.
5miles - I'm confused by your statement that UK airspace would have to be resectorised.
If the QNH Zones are aligned to TMA boundaries, there shouldn't be any issues. I'm sure the one or two hPa difference between adjoining TMA's isn't going to be an issue. It works in NZ, and at this time of year there can be a 30+hPa difference over a distance of 50-80NM.
The only issue for Area sectors, will be having to issue different QNH settings as aircraft transit between QNH sectors.
10W - Hopefully the above has given you some insight as to how it all works in a country about the same size and shape as the UK. Everyone in the same bit of airspace gets to be on the same altimeter setting - it's not Rocket Science
. Travelling 600NM on your departure aerodrome QNH is just a bit far fetched, and in NZ might result in meeting with some Cumulo-granite.