zkjaws, you answered your own questions!
If the QNH Zones are aligned to TMA boundaries, there shouldn't be any issues
That is the first problem that will need sorting out.
The only issue for Area sectors, will be having to issue different QNH settings as aircraft transit between QNH sectors.
Unfortunately not, there is also the issue of what the new TA will be, and if 18000ft is where its at, some sectors that are vertically split will lose half their levels when there is a low QNH (the majority of their sector is currently FL180-FL210, and they have standing agreements at FL180, FL190, FL200 and FL210 which all separate from each other). As the London TMA is quite a compact and complex piece of airspace, whatever the level is below 22000ft will mean rejigging which won't be a simple fix. You can give me almost any level, and I can give you a change that means lots of work.