In BA there is a system of sorts (I'm talking mainline).
Flight numbers 1-299 = Long Haul, 300 + = shorthaul. 1000 series = LHR domestic, 2000 series = LGW. There are more for different franchises, codeshares etc.
Interestingly, LHR SH = even numbers outbound and odd inbound. In LH this is the other way round, ie odd out, even in. This is baked in history, I think, dating from BEA and BOAC days.
There are other rules of thumb, 300 series flights tend to be France, 400 Spain, 500 Italy, 700 Austria/Switzerland, 800 Eastern Block, 900 Germany.