Hi Dar,
I'll take a stab on this...
There are certain agencies which look after overseas contractors and their various tax situations etc. I'd have thought that you sign up with them and they deduct whatever is required from the contractor pay/tax situation you're in leaving you with your 'take home' dosh.
I think the legislation which covers this is called IR35 - I'm not 100% sure it's what will affect you but I do know it covers 'contractors' who work in UK companies (even if that company is a subsiduary of a US one)
Now, another route would be that you have your 'own' company - maybe form it there in the USA and then you 'transfer' yourself over here. I'm thinking more along the lines of creating a legal way in which to work over here and one which covers Visa's as well. I'm not sure if a non UK citizen can 'setup' a brand new company here or not - I know I as a UK citizen cannot 'seup a new' company in the US.
Just some points to think about and maybe research. I do remember seeing adverts in my weekly 'Computing' magazine (which you can get from WHSmiths here) aimed at international contactors working in the UK on work permits etc - I think it sounds like you'd be one of them- I'll try to see if I can find the name of the agency, hope they advertised in the last 2/3 issues !
As always, speak to a qualified UK/US legal person who knows both systems in order to get the correct advice..
hope this helps a bit!
Cheers, DtM! West London & Slough UK!