When recommending BP monitors I always recommend Omron and advise buying one with an upper arm cuff rather than an wrist one. Some years ago I brought my first BP monitor, a wrist one. When a few weeks later I checked it against the GP's it was way out. The extremely pleasant GP wrote a note there and then saying it was not fit for purpose especially as I was over forty (as arteries may harden/calcify). The pharmacy swapped it out for my first Omron. My current one is five years old and checked out against the physiotherapist's NHS calibrated one two weeks ago.
Recently a friend spotted mine and I offered to check his. He said his was fine as they had one at home but agreed out of interest. Over three readings five minutes apart it averaged out to 153/87 being about 28/15 over what he expected. I suggested he buy a cuff model and chats with his GP but I am unconvinced he will.