One big problem
I thought I'd go take a look a this. One big problem -- Unity Web Player is only for Windows and Mac! This is the point where someone would make a snarky comment about Linux having "no" market share or something. 1) Simply not true. 2) This GCHQ-backed site is meant to attract people into infosec, I would expect the demographic drawn to this site to be significantly off the norm in terms of both OS and browser usage.
edit: Per Google there's kludgey way to make Unity Web Player work as a firefox plugin, that may or may not actually work. But still, I would hope something brand new would just use HTML5 or something.