There are many vendors offering a similar watch for £18 or less, so do not pay £35. you can get a toy watch for less. And they are toys - the blurb I have seen shows the target audience as being children.
The technology either measures skin colour or sweat and purports to measure glucose levels. This technology has not yet been successful, and one poster on the forum found tht the results from their wqtch was 50% out compared to a fingerprick test. You will note thst the sales spiel for these watches make no claims for accuracy or refer to any standards such as the ISO that genuine glocometers need to meet. I also doubt if they are CE marked, which is why they are being sold in post Brexit Britain.