In my opinion the "flight connections" route is definitely the one to go for (although the signposting IS terrible and the empty corridors make it seem as if you've taken a wrong turn :confused: ). The queues at security are not so horrendous and if you need to check in at the BMI counter I've always found it (along with BA) to be overstaffed. Immigration only has one or two lanes, but in my experience seems to move quite smoothly (although I am a British citizen).

Terminal transfers at LHR are always going to be six of one, half a dozen of the other. Sometimes you'll get through faster one way, other times faster the other way. In fact this is true at almost every airport.

The only advice I have is that unless you're really stuck for time (in which case, ask flight staff or gate staff for advice), go the way that the airline recommends. It saves awkward questions about why you're coming "back into" the airport even though you're transit. I've seen people held up a few times while the security staff work out what is happening.