Where is the best place to purchase these as it seems a minefield out there, prices seem to vary so much too and don't want to be fleeced
If you can afford the Garmin GB TOPO Pro buy it from somewhere reputable as the cheapo copies on flea bay etc may have poor quality map tiles, contain outdated mapping and may not load in your device or all of the above
. If you do risk buying one, make sure they have an effective returns policy for a refund!
The OS online shop has this at an eye watering £334.99
As sussamb says current free mapping based on Open Street Map is fairly usable these days e.g. OpenTopo map.
https://opentopomap.org/#map=13/54.16555/-2.39399You could also take a subscription out to OS maps and use it on your phone as a back up and to print out map sections. You could also grab a screen shot of a map section on Bing, georeference the image as an overlay in Google Earth and then save and upload it to your Garmin as a KMZ custom map to navigate.