Basically all HRM’s will include the current time, and allow you to start a stopwatch and check out the current heart rate (OK, there might be really cheap models which only show HR, but spend 25gbp and that shouldn’t be an issue). I’m not aware of any accuracy issues, I think all HRMs on the market correctly measure HR.
Beyond that, the following features may or may not be of interest:
* Fabric chest band (more comfortable, but the plastic ones aren’t that bad)
* Changeable chest band battery (the cheaper Polar transmitters won’t let you change the battery, meaning you have to buy a complete new band… not cheap!)
* Waterproof
* Min/Max intervals (with some kind of audible alarm when you’re not within that range)
* Encoded transmitter (useful if you go to a gym, it means you’re not receiving someone else’s data)
* Bluetooth monitor can be connected to phone, but won’t be compatible with gym running machines (etc.)
* Data upload to a computer
* GPS, temperature, ant+ compatibility, etc. etc.
If all you want is the most basic stuff, the Decathlon ones are fine. I’ve had problems with them in gyms, despite supposedly including encoded data transmission, but I might just have got a flaky model. The Garmin models are well thought of, too.
I currently use a Polar, but I’m not too happy with it as it won’t show the stopwatch and the current HR on the same screen, which is a pain when you’re doing intervals. That said, Polar has an excellent reputation and it’s a user-interface issue, apart from this niggle the HRM has been fine.