Answered my own question via this website:
http://netflixukvsusa.blogspot.co.uk/2014/
While US customers get access to over 10,000 films and TV episodes for their $8 (currently worth £4.87), we get less than a third of that for £6.
Just to rub salt in the wound, some of the stuff they get that we don't were ours to start with, such as loads of classic Dr Whos.
I shouldn't be surprised. It's already normal over here to have new releases at our cinemas that are already on DVD and BD in the States.
And then they wonder why illegal downloading is so rife.
Not that that's linked to anything to do with Netflix, but it's all par for the course over here.