"This seems to be a product of al Qaeda's desperation to try to get an attack through, but it's a potentially scary development," Raffaello Pantucci, a senior fellow at the Royal United Services Institute, who closely followed the trial, told CNN.

While Islamists militants have access to many bombmaking recipes online, analysts say they often contain potentially fatal errors and have been seen by militants as a poor substitute to hands-on training by an instructor schooled in the art of bombmaking.

Security services fear the presence of such instructors on Western soil could be a game changer.

Message from al Qaeda: Stay Put

The Birmingham case revealed that al Qaeda had begun actively dissuading recruits from going to Pakistan.

"They told me don't send anyone [more to us]," plot ringleader Naseer said in one of the recordings.

In a June 2011 video "You are Only Responsible for Yourself," released around the same time the Birmingham duo trained in Pakistan, al Qaeda's leadership in Pakistan had urged Western militants to stay home to launch attacks.

Khalid, in one conversation recorded by British police, pointed out that Inspire magazine -- the online English publication of al Qaeda in the Arabian Peninsula in Yemen -- had the same advice: "we don't want people to be traveling cos it's getting risky." In 2011 Samir Khan, an American member of the group, urged followers to launch attacks in the West rather than travellng to Yemen.

Naseer and Khalid painted a dim picture of the tribal areas of Pakistan in their attempt to stop their associate from traveling there.

"[Waziristan] hasn't got no more camps now... the brothers used to be in the mountains [but] the drones just get them straight away, they just bomb the camps, so ... they taught us inside houses," Naseer is heard telling the associate in the recordings played in court.

He described how he had had to cower for several hours in 100-degree heat when a drone circled above. "That day was bad ... it's a nasty situation to be in, man," chipped in Khalid.

Khalid described how the drones restricted movement for fighters in the region: "They were restricted to one place most of the time: One place to eat, sleep, go to toilet and do everything."

Terror training on UK soil

Naseer and Khalid implemented the new al Qaeda strategy when they returned to the United Kingdom. Just days before their arrest on September 18, 2011, Naseer provided Ashik Ali, the third Birmingham man convicted Thursday, with hands-on instruction in how to make explosives.

According to expert witnesses consulted in the trial, Naseer had acquired the correct knowledge to teach recruits how to make viable bombs. The recordings suggested their planned attacks were still months away.

In one of the bugged conversations, Naseer is heard suggesting that the restrictions on training in Pakistan meant it was just as effective training recruits in the West.

"[In the tribal areas] you get bit of an experience in fighting -- but you know the rest of the stuff that could be taught -- they taught us in a room."

At one point, Naseer warned Ali that if he hit or rubbed the explosives in a certain way it could explode and potentially kill him; just one of the many lessons he passed on from his training in Waziristan.

In the days before their arrest, Naseer experimented and tinkered with potential bombmaking chemicals in a Birmingham home, including the chemicals inside sports injury cold packs. The recordings revealed that his instructors in Pakistan had taught him that ammonium nitrate- - a potentially high powered explosive -- could be extracted from such packs, demonstrating al Qaeda's continued inventiveness.

Pantucci told CNN the Birmingham case illustrated that despite intensified drone strikes in Pakistan, Western militants were still finding ways to connect with al Qaeda in the region, providing the terrorist groups opportunities to attack the West.