You guys may laugh now, but give video games a couple of years and even you could be fooled by their realism. Take a look at this fan film, for instance:
These video editors only used video game footage as the background, and kept real actors in the foreground. Games like Crysis already have amazing environments, but just imagine how good they'll look in a few years. Heck, isn't modern CGI already convincing? Take a look at this video while you're at it. If we can move beyond using limited polygons for 3D models, the level of realism will shoot up immensely. Further developing this technology is a simple matter of resource and production planning on the part of our computers.
The other point people are missing is the possibility that ITV was using the footage as a cheap, dramatic reenactment. Sure, it's really cheesy and fake now, but it could be a viable choice in the future.
