Ref the 3 readings, or 24 hour BP monitor: at the last Class 2, the BP was a bit high. The reasons for this turned out to be an issue other than white coat hypertension - now sorted -, however the AME discussed with me future Class 1 issue and what it might mean.
He said that he could get me on a 24 hour monitor prior to the Class 1 initial, which I could then arrive with at Gatwick if the BP reading did turn out to be high - 6 months to get an NHS one sounds excessive, perhaps due to the area you are in, but I would suggest, if you are going to go for the 24 hour one, to go via the AME rather than local GP (or ideally get a GP who is an AME!).
The monitors, it is fair to say, are not most accurate, however some such as the Braun are nowadays more accurate that NotSo suggests - perhaps times have changed, but of course for the ideal reading the surgery equipment is best. Perhaps take the electronic one along for a reading at the same time as the GP takes one to judge how good it is?!