I've been hemming and hawing for several hours now because i haven't been using MO2 long enough to figure out the correct procedure to keep both games separated and i'm worried that in my inexperience i might mess up my existing Fallout 4 mod list.

So could someone kindly explain step by step how to install Fallout 4 London with Mo2 without breaking my current Fallout 4 in the process? Also, should i do this through a new profile or instance?

Yes make a new profile, and check both "Use profile-specific" boxes. Afterwards, go to the Fallout London install folder and zip archive the whole data folder. Then in MO2, click on the "install a new mod from an archive" icon second from top. Navigate to that data zip archive, install the mod, and now you're good with keeping both games separate. Also, copy and paste the original London ini files into the new profile. Do this by going to the Fallout London install folder, then into the __Config folder. Copy both ini files. Then in MO2 click on the folder, (2 icons right of your profile) and click on Open INIs Folder from the drop menu. Paste and overwrite the ini files there.