It's not that your Miranda/Jack scene idea is bad Flog, it's great and would look awesome. But it would require tons of resources and developement time. Mo-capping, animatins, music, camera angles etc etc. Not to mention the issue of voice acting. And that's just ONE War Asset scene. Now imagine creating just as elaborate cutscenes for everyone else. There are tens of important War Assets and tons of variables for them. Not only would it take months and huge amount of resources, but it could also transform the London mission into a movie rather than a game. The gameplay pacing could become way too chopped up. Too much cutscenes ain't good either.
Personally, I think the best, most realistic, and most efficient from resources POV way to do it would be having the models of our War Assets be placed around London map with some combat animations.
Something like the Asari soldiers fighting on Thessia, it could be Jack and her student making biotic barries for Alliance soldiers standing on baricades in London:

A little bit later, we could see Ex-Cerberus soldiers (models taken from multiplayer) along with Miranda & Jacob (if they're alive) fighting as well with some standard combat animations.
Further down the road, we could pass by (or see a bit in the distance on some barricade) Grunt & Aralakh Company along with models of Rachni troops fighting together. Maybe we could even see Grunt dying (standard 'getting killed' animation) if Rachni Queen was not saved.
Then would could walk by Vorcha, Blue Suns & Zaeed.
Then we could spot Drell Assassins with Elcor Tanks.
Just that. Our assets in London without elaborate cutscenes. I just don't believe there's any chance for elaborate cutscenes for War Assets. It would take too much time & resources for Bioware, EA would not greenlight it.