If you have the OS maps on your Garmin, use BaseCamp which runs native.
Easiest is to have the GPS connected via cable, and BaseCamp will “see” the OS maps.
Fastest is to copy your OS maps from the GPS to a disc image (which BaseCamp will think is your micro USB in the GPS, but will transfer maps to the programme much quicker – you’ll need to unlock the IMG file you copy over, but a quick Google should help there).
Rarely if ever use Memory Map since I got that sorted.