Bookbinder's link will require you to pay £1 for a month's trial membership in order to read the reviews.
Personally, I think that Which magazine has had its day. You can learn just as much about a product (from people who've actually used it, rather than testing it in a laboratory situation) through seeking out reliable online reviews.
In particular I like sites such as TechRadar which, when combined with product reviews on Amazon, can be a genuinely useful source of information:
https://www.techradar.com/uk/news/wearables/10-best-fitness-trackers-1277905
(Further, such sites are frequently updated, whereas Which tends to wait several years before reviewing each type of product again).