Because nothing says "self-defense" like bombing forces of a government whose country is 3600 km away from your home country
The British military which has inserted itself into Syria illegally and against the wishes of its internationally recognized government bombed and killed the soldiers of said government as reported by The Sunday Times citing a spokesman for the British Defence Ministry.
The British Defence Ministry says a British Eurofighter jet dropped a 250 kilogram bomb on Syrian troops when a desert battle erupted near the British-American-rebel base in Al-Tanf in Syria:
“As an act of collective self-defence, RAF Typhoons dropped a single Paveway IV on the position, which successfully removed the threat to our coalition partners,” the spokesman said.
A Syrian army officer was killed and seven others were wounded, according to local reports.
The phrase "coalition partners" means this was a clash between the Syrian army and the rebels, not the Brits, if indeed there was such a fight as the British claim at all.
In 2017 the Americans repeatedly massacred Syrian soldiers from the air for simply coming within 55 kilometers within Al-Tanf -- this is the extent of the arbitrary zone around it which the US has appropriated for itself -- in someone else's country.
London already participated in Trump's April strikes on Syria and has been a key backer of the jihadi-fundamentalist rebellion in Syria since 2011.
What are British soldiers doing on Syrian soil in the first place? Imperialist meddlers out.