As an International transfer, the process is fairly easier, but the above posters are correct. You'll have to stay within the airport (and there is nothing outside the airport), and you should have your boarding pass with you.

You'll most likely see the Customers B section on your left as you've exited your flight in the US Arrivals section, and then its an easy venture to the area of the airport where your LHR flight is taking off from.

Its usually a fairly painless process, especially if you have your boarding pass and customs document filled out properly - there is usually very little hassle. I've never spent more then 45 seconds at Customs B when I've flown SFO-YYZ-<INT>.

If your United membership is *G, then take advantage of the Maple Leaf Lounge in the International departure (or pay the $25 depending on how long your layover is between flights). Free booze, good snacks, free wifi, showers. Its worth it, especially compared to other crappy North American lounges (like the Red Carpet club).