if water isn't coming from the block and the oil isn't emulsifier then you'll be good. water doesn't boil in the system because its under pressure, when its not it will boil. it took me ages to bleed out all the air in the system.
1. you say water was leaking from the block before the HG change. was the head gasket skimmed or checked to make sure it was flat and not warped before going back on ? excess heat will warp the heads on these.
2. when you did the heads did you pull the water pump ?
You got it to the MOT station, it went through an MOT and emissions test without overheating but it overheated on the way home so i would say its holding ok, you just need to keep burping it. you will probably find that the coolant level will work down over say 200 miles and keep topping it up as the airlocks work out. the heater matrix is, as BTD says, the big one.
You have to remember that the RR is a heavy, luxury 4x4 with big lump under the bonnet, it's no Ford KA.
you're getting there..