If your flying VFR, you use Regional QNH, and you dont use the Quadrant rule. Even if your above the TL.
I don't. I fly at FL and try and fly quadrantals. The reason for this is two fold - 1) most upper airspace is defined by FL (i.e. airways) and this ensures I don't bust them, and 2) If I happen to meet IMC conditions en-route I can continue IFR with minimal fuss.
Altimeter subscales are just numbers. It doesn't matter whether it is 2992 or 1013 you just twist the relevant number in. Same as if your ASI is marked in knots, MPH or KMH, you just fly the numbers and don't even bother trying to convert them, so no big deal.
I think a TA of 18k would be good because it would bring Europe in line with the USA and it would ensure clearance of all of Europe's highest mountain peaks. I'd then suggest that the UK drops the Class C from FL195 to FL180, just so we know where we stand with regards to upper airspace.