Ex EUR means exactly what it says.
You have to get to a European airport and ‘start’ your journey there.
As a European based traveller, this means I automatically get a better deal than you, as I do not have to travel abroad before starting the real journey.
In the whacky world of BA, seems that the UK is not in Europe.
When I used to live in the UK, I used Lufthansa extensively for long haul, as it seems that they like to give better deals abroad, so long as you don’t mind flying indirect (and that adds one extra flight, rather than two to go to Europe, return to UK, really start your journey), then I’d look at LH and LX next time.