Ok, its hard to speculate from all ive seen of ME3 which consisted of a destroyed earth and soem british guy. Now, what Im thinking is that this british guy has little or nothing to do with the plot and its just a vehicle to show the effects of the attack on earth.
Perhaps the brit is there because he will be a secondary primary char (sort of like Laguna in FF8) to which you follow his plot for a portion of the story on the ground in defense of earth. What seems feasible to me is if they are going this route what this can do is set up a vehicle for a ME spin off to be a true shooter title where your the brit who is fighting in defense of earth. By introducing him in ME3 it seems like an attempt to craft a solid char to build around as opposed to introducing one unheard of.
Again if they are making this guy a player in the story, its entirely possible that there could be something that has blocked off shepard from helping, and the first portion of the game is centered around from freeing shepard from this roadblock and once you do so, the brit becomes a primary companion of shepards party and shepard takes its role as party lead.
Honestly, its hard to speculate anything based on one and a half minutes of video. I think the best thing is to have patience and wait till either more is revealed or just wait till its released which is what I am trying my best to do.