A direct quote from the website of our local GP's surgery:
"The Practice is encouraging all patients to purchase their own personal items for use such as:
Home Blood Pressure monitor - Pulse Oximeter - and Digital Thermometer
These items are inexpensive and widely available from any pharmacy or online. They will not only be a useful addition to the home first aid kit but play a vital role in self managing healthcare issues and will enable you to report accurate readings to healthcare professionals either via our online services or during telephone consultations".
This is the blood pressure monitor I use daily:
https://www.ebay.co.uk/p/501438974
(I bought mine for under £20 from Lidl when they were having a promotion on health products). The readings it gives seem consistent both with those that I get from the mega-expensive electronic units that they use at Ipswich hospital and with the old-fashioned manual method used at my GP's surgery.