No sure for LHR but I travel regularly for biz between DAR NBO and DXB

One hour Flights from DAR-NBO return are $400-$500. Flights from DAR-DXB are $650-$700...So I always take a KQ flight to DXB and either on the way there or back deliberately book a long layover of over 12 hours. The ticket always says that I have to pay extra, but having a boarding pass for the 2nd flight I just walk out in NBO - finish my meetings and return in time for the 2nd flight.

I am not even charged the NBO tax which you are supposed to pay if you land in NBO...and my Tanzanian passport gives me visa free entry to Kenya...so no immigration formalities either...