It's probably just an academic question because they've all been taken.
If you search for something like 9r.org.uk you'll notice that a lot of 2 char .org.uk domains are still available and you just register them in the normal way. I imagine that if you spotted a 2 char .co.uk that had dropped and that nobody had snapped up that you would register it in the normal way too, i.e. using a registrar like 123reg.co.uk