That's pretty much the worst part of the experience, I think. Any way you look at it, you have to go through security. I'm told that it's also possible to clear immigration/customs and then walk over to the other terminal outside, instead of going through the flight connection center.

Overall I find the connection experience at LHR pretty unpleasant and time-consuming. Figure on a bare minimum of 30 minutes by the time you walk from the gate, get to the bus, get off the bus and go through security.