The 'load other door marker' option requires you to have access to the marker file on your system.
As the UK library is probably an LCF (Library Container File), you will need to extract that file so you have access to all of the individual files.
Then you will be able to load and browse for that marker file.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10