More than 10,000 documents and a huge amount of digital data were seized after the group was driven out of Manbij in northern Syria in August, according to Major General Rupert Jones.
“If we want to keep Britain safe, we need to deal with Daesh,” he said, using another name for the extremist group.
The news comes as anti-terror police have started deploying on London’s streets.
General Jones said: “External operations have been getting orchestrated to a very significant degree from within the caliphate, critically from within Raqqa and from within Manbij.
“They were key external operations hubs. There is a huge amount of intelligence, documentation, electronic material that has been exploited there that points very directly against all sorts of nations around the world.”
He declined to discuss details of the suspected plots as he spoke to reporters at the Al-Assad air base in Iraq.
British security services are analysing the material.
General Jones said they will be expecting fresh intelligence if the coalition retakes the Iraqi city of Mosul, where US-backed Iraqi and Peshmerga forces have launched an offensive.
“It will be a labyrinth of intelligence and we need to get that into the hands of the intelligence agencies,” he said.
Attacks either perpetrated or inspired by Islamic State have struck cities across Europe, including Paris, where 130 people were killed in November last year, and Brussels, where 32 people died in March.
Last week, French anti-terror police foiled a terror ring plotting attacks in France.
Seven people were arrested of French, Moroccan and Afghan origin in Marseille and Strasbourg.
French interior minister Bernard Cazeneuve did not reveal the intended target of the plot, but did say “the foiled attack was a co-ordinated attack aiming to target several sites simultaneously”.
The US State Department urged Americans in Europe to be vigilant against a “heightened risk” of terrorism during the Christmas period.