Sounds like you need to learn not to touch your navigation device while driving - whether it's a phone in navigation mode or a dedicated navigation device.
If you find it's feasible to get the UK maps loaded onto that Garmin, just be sure the map install was successful after it's done. I have a good old Garmin (since discontinued but still supported because it came with lifetime map updates). Now and then I update the little thing, especially if I'm heading off on a road trip - although these days I find Google maps on my phone to be sufficient for most sat navigation. A couple of times I've run the update on the Garmin, which was supposedly successful. Then gotten into the car, turned on the Garmin, and gotten a "can't load maps" or similar error message. The cure was to hook it up to the computer and reload the maps. Not a big deal when it happened before I left on a trip. More inconvenient when I first turned it on after I'd hit the road.
Personally I still prefer the Garmin interface to the maps on my phone. Somehow the Garmin map/route displays make more sense to me, and it usually displays the speed limit of the road I'm on which I find very useful.
And, I'd like to know more about how the police in the UK ticket drivers for touching their mobile devices. Is it OK to touch the device while stopped at a traffic light?
EDITED to add: Okay, I asked my good friend Google. Wow. This is serious. You CAN'T touch your mobile device if the car is even running. Good for the UK. https://www.thesun.co.uk/motors/2180138/illegal-mobile-phone-driving-laws-fine/