It works with a HR strap, I use it all the time.
Apparently you can connect other meters, and you can get third party apps to display the data but not record it. Not tried cadence.
The mapping does include UK and is pretty good. They are vector maps unlike the OS ones, and they are sometimes actually better than OS because there are more trails. Round here they also include some (but not all) of the cheeky homemade MTB trails. Routing is now good, unlike the older Edge Touring.
You can also change the amount of detail visible on the maps, which is useful, and you can route to addresses and POIs which is occasionally useful. I plan routes on Strava and there’s an app on the device that just syncs with them. I’m dead happy with it as a nav and recording device.