Twice daily readings (UK)

So I'm having to do these again. We've all been there, right?

I was instructed to provide the best result from each of two daily sittings: morning and evening. So far these have been ok.

But is that really sufficient? What if your BP is really high the rest of the day? Just for curiosity's sake I took a reading about half 1 pm before I had my lunch, and it was about 158 sys! That seems really high (i know it increases during the day).

Is this really an effective way to diagnose hypertension? Maybe it's different elsewhere but that's the norm here in the UK.