Yup, the real ATMs are gone, replaced by what they call Cash Machines run by Travelex--same crappy ripoff rates as at the money exchange booth at Seatac. All the Heathrow Terminals have both live Travelex and/or Amex currency exchanges booths or the machines in the arrivals hall--makes no difference in the rate you pay. So you could just grit your teeth and get the money you'll need either at Seatac or at Heathrow, knowing you're paying a ridiculous markup and accepting it as the price of convenience. I'd do that if it was a small amount, but a cab ride is likely to be a pretty hefty amount.
If you want enough sterling for a cab ride from Heathrow into town, you're better off going to a main branch of your bank and getting sterling there. Still more than at a regular ATM but much less than at the Seatac or Travelex booths. Bank of America's downtown branch actually has pounds right there--my local branch has to order them. So check around and see whether your bank can get you set up before you go.