On departure from Heathrow (much like arrivals into Heathrow), aircraft have to navigate some very complicated and busy airspace (with flights in and out of LHR, LGW, STN, LTN and to some extent LCY all competing for space in the lower/mid levels, plus many flights inbound to CDG, BRU, AMS at higher levels preparing to descend) - based on a lot of different factors, of which final destination or mid- to long-haul routing is not really one, aircraft heading East will do so either to the North or South of London itself - there is no obvious pattern to it in terms of final destination