Personally, I'd not trust that one either, not without a stated R-value.
There's a decent write up on R-values here, pay particular attention to which standard manufacturers are using. As a decent rule of thumb, for year-round use in the UK (assuming you're not doing winter mountaineering in Scotland), an R-value of 2.4 or more using ASTM FF3340-18 should be fine (4 or more under the old rating systems).
If you're only planning on milder weather, you could consider a less effective mat doubled up with some CCF, but this will add bulk and faff.
Bear in mind that you'll pretty much never have a mat that is "too hot", unlike sleeping bags where you wouldn't want to use a 4-season bag in the height of summer.