Though the penny does not seem to have dropped, it has long been the case that considerable savings can be made by booking BA flights exEUR. This is no revelation, nor is it news.
The same is true if you were a German national looking to book Lufthansa from a non-German airport etc.
The UK’s lack of competitiveness is further exacerbated (especially on ultra longhaul) by the sizeable cost of Airline Passenger Duty levied by the UK Government, which automatically increases the cost of exLON fares, regardless of airline.
In assessing the differential, you should also take into account the extra time required to reach the European starting point, the cost of the flight to get to your starting point, the possible need for an additional overnight, the risk of baggage misconnecting and the general hassle of it all.
When choosing carriers other than BA there is in addition the often better hard product aboard BA (lounges/fully flat beds/arrivals facilities),
When you add those actual and perceived “costs” to the fare, and deduct the 10% discount available to IAG shareholders (and likely soon to be extended to Iberia flights), a BA exLON *sale* fare can often be more competitive than you might imagine.
Great result on saving £3000+, though!