From The Guardian
Boris Johnson hit an immediate hitch on his first assignment as foreign secretary when the plane carrying him to Brussels for talks with EU officials was forced to make an emergency landing.
The Foreign and Commonwealth Office confirmed that the aircraft, believed to be an RAF passenger jet, a BAE 146, experienced a technical fault – thought to be an issue with the plane’s hydraulics - and had to land at Luton airport on Sunday.
Flights to and from the airport were grounded while emergency services dealt with the situation.
Or he could have gone London - Brussels by Eurostar.
e.g. ETD London 11.04 - ETA Brussels 14.05 and then every hour
It's quicker by train
..or what's the saying "If you've time to spare go by air"