The thing is that not all slots are created equal. A late northbound from ,LHR may not be that difficult to get slots for, LHR is quiet at that time and late departures are not very useful for long-hail. The early morning arrival would be more difficult, it's peak arrival from North America and Europe.
There are quite a few longhaul departures in the evenings: overnight flights to places as diverse as South America, Africa, India, Kazakstan, Asia, and Australia, and not just on BA/VS.