Why would the Brits leak such info, when the effect is to make them look inept (can’t even control their own Ambassador)?
Maybe they didn’t. Maybe someone who wants this ambassador gone did.
Lots of possibilities.
Could be someone who has a downer on the Ambassador and wants to make him look ridiculous.
Could be someone who covets his job, and thinks this will get him out of the way.
Could be a foreign power. The Russians perhaps. Or the Chinese. Maybe even the Iranians. They all have the capability. Lots of other countries gain from a wedge driven between the UK and the US.
Could be leftists in the UK establishment. British leftists are like American leftists. A lot of them are anti-American on principle and very anti-Trump in particular.
Could be extreme EU fanatics. The pro-Brexit people count on Trump and the US picking up the slack on trade when Britain leaves the EU. If Trump gets upset with the UK, that makes a no-deal leave of the EU less likely.
Whoever did it, they’ve gone to a lot of trouble. Some of those comments go back two years, and British diplomatic communications are destroyed after three months. Someone has been hording these reports (or more specifically the juicier ones) for a long time. I don’t think its a coincidence this has happened after a pretty successful State visit by Trump to the UK.
In any case, we need to know who did the leak and why.