We were connecting at the new Terminal 2 from United to Swiss on the way to Zurich. I don't think I've ever seen a larger terminal that required that much walking! First, even though you've already been through security to get on the plane and you've only been in a secure part of the airport, you will need to walk all the way from your gate to ... Heathrow security! They're about on par with the TSA in terms of degree of difficulty so just follow all the rules posted and don't get creative.
THEN, you will have to almost exit the terminal and walk ALL the way back towards your gate, if you are lucky enough to know where it is. Swiss didn't post our gate until 20 minutes before the flight, and of course it was all the way away from where we were waiting. The Brits know the size of the terminal is an issue, they have little warning signs posted near the direction signs telling you in advance that where you are hoping to go is a 10 or 15 minute walk (if you hustle).
That said, the shopping and eating options in the center court are very nice. Got my mom a gift from Harrods so got major credit for the forest green logo bag. Be sure you have your boarding pass handy since you won't have to pay VAT if you can prove you're just passing through.