Hey J, I certainly think we've always followed the LHR Flight Connections signs - they've always looked to me like a guy with moose antlers, but are really of a stick figure walking along, with landing and departing planes aiming at/away from his head.
The escalator just past passport control that Zeeba mentioned above can be tricky. Because that Terminal 5 Security is one floor up, immediately after the escalator, if there's a backup at security, the line of people being transported up the escalator can get dumped on the heels of the people standing in the security queue. Two years ago, an airport staffer was holding people up at the bottom of the excalator for a minute, to allow traffic at the top to clear out, and security staffers were trying to spread out the queue at the top; but the last time, there was no one doing crowd management, and when we got to the top, we had to do a little sidestep (there's not a lot of room at the escalator exit up top) to keep from rear-ending the people who'd stopped once they'd stepped off the escalator. Hopefully the crowd keeps moving.
Not sure about Terminal 3, if that's where you'll be, but still look for an "Eat" food vendor. We got food from Eat at both Heathrow and at Turnhouse airports in Edinburgh this past August, with quick, quality offerings at not-too-expensive airport prices for what you get.