The things they claim to be “leaks” appeared in their own press and other European press, and I think the claim that the US did it is just an attempt to cover up their total incompetence in preventing terrorist attacks even though they actually have intelligence on these threats. They want to cover up the connection with Islam and with “refugees,” and they were probably hoping they could continue with the ridiculous meme that it was a “popping balloon” or just a random explosion of some sort.
Trying to deflect attention to us, however, has given Trump a great opportunity to look deep into the FBI and find out what the heck is really going on in that politicized and incompetent organization. Like its Brit counterparts, it has also investigated and followed virtually all of the terrorists involved in US attacks, and has even gotten foreign intelligence on them - and done nothing until dead Americans are lying all over the place. So yes, there’s a problem.
On the contrary, the successes of the British police in preventing attacks outnumber their failures by at least 90+ to 3. (90+ is the number of convictions in the last three years alone for conspiracy to cause acts of terrorism - ie attacks planned but foiled before they could be executed. 3 is the number of 'successful' attacks - ie resulting in fatalities - in the 12 years since the London tube bombings).
Given that no police force in the world has ever been completely successful in preventing all attacks by a determined terrorist group, that's not a bad record.