I don't think you'll have any problems with the 2 who are not travelling. If it were LGW I'd liaise with the ticket desk to advise the others weren't going and to check if there were any issues regarding your return flight. I would imagine LHR will do the same or send you to Zone E to check.
You'll only be able to check in the people travelling. Even if you check in one of the people not travelling, they'll be offloaded as soon as they miss conformance at LHR so the seat could be reallocated.
I'd do OLCI and get the best seats you can for the two of you.
Have a good trip ^