In reply to Russell Lovett:
I literally just did this (we probably got a similar deal on the watch!).
I spoke to the guy a fair bit about the maps on whatsapp and he is very responsive and helpful.
I paid for the one off maps which were about £11, (the ones that include the elevation data, think they're called topo+ or something). I didn't think I'd benefit from the annual subscription.
They downloaded immediately, I plugged the watch in to PC, showed up as a USB device, I copied the map file over (after unzipping), then changed the map selection in the settings as instructed. Whole process took <10 minutes and was very easy.
The maps are amazing, basically look like OS maps, have street names etc. as well.
I then went and did a 50km run/hike in the peak district and they worked faultlessly. You can see streams and bogs and contour lines and cliffs, paths, roads, summits - but also "location names" and summit names which is really helpful. I find them an absolute game-changer compared to the basic breadcrumb navigation line in a blank space on my old 735xt. Can't recommend them enough.
As an aside - stupid that the screen protector film that the new watch comes with shows the maps with contour lines and rivers when that's not what the watch actually includes. Just happy the TT maps are so cheap tbh.
For routes I use OS maps hugs online and just download the gpx file and then upload it to the Garmin connect website and then sync to watch, it shows up as a line on the map (in addition to your actual route taken line).