Yeah. I understand. Split the difference. Make it look just enough unlike a possibly-protected map to make that a non-issue but clear enough to be playable. Maybe less than 120 terts? That many would place it 240th out of 255 in tert count. The most popular games are smaller; often MUCH smaller. Consider mapping only Zone One. You'll have more room for some cool London graphics. And though Londoners will be upset, most of the rest of us are at best familiar only with the central part of the city. And then only bits of it. Make a map that 1) is a good play, and 2) teaches us about the geography of
central London. In fact, as an alternative, consider a map of London without bothering with the underground, or use it as just one feature.
I just google image searched "tourist map london" and found
https://mapcollection.files.wordpress.com/2012/07/london_centre_road_-_tourist_map.jpg among many others and boy, you could make a pretty cool map out of that unbelievably historic bit of geography.
How about a game entitled, "London: The Blitz" where the Luftwaffe is a few key terts off to the side that can bomb the whole city?