Find a friend who is coming to the UK.
Phone Andair direct. Its easy to do it that way and really is not expensive. Pay for the valve using your US credit card. Andair will be happy with it.
Get the valve and extension delivered direct to wherever your friends are staying.
I'll be surprized if ACS or Vans will be cheaper than Andair direct. If you can avoid shipping and any customs charges you should come out ahead.
The dollar is now up against the pound by nearly 25% - today I think the rate was GBP1 to USD1.56. I managed to buy my kit and other parts at better than USD2 to GBP1 which made me happy -



now the rate suits you guys much better.
Hope that helps,
Chris