I'm not from LHR but, for what it's worth, I don't think there are any hard and fast rules on this - it's up to the pilot tof the second aircraft to line up when he/she considers it safe to do so. I would have thought that it would be good practice to let the jet blast/prop wash to settle before lining up but that's just me I guess. In most cases (with transport type aircraft) there shouldn't be that much rush because there'll usually be at least 1 min delay for IFR separation.