Sahayb Abu of Dagenham prepared to strike last summer but was arrested after discussing guns with an undercover police officer.
Pc Ben Hannam, 22, had also pleaded guilty to possession of a prohibited photograph of a child.
The male officer was not charged or suspended over the 2017 allegations.
A father-of-two was found dead in a pool at the entertainer's home. No one has ever been prosecuted for his killing.
The rail worker died of coronavirus in April last year, two weeks after allegedly being spat at in London's Victoria station.
David Chambers pleaded not guilty to two counts of fraud and one count of battery.
The victims had hoped for a better life in Britain when they agreed to pay up to £13,000 a head for a “VIP” smuggling service.
Pawel Relowicz, 26, is on trial over the student's death in February 2019.
The incident took place at a care home in south-east London in the early hours of Monday morning.
The ruling has been described by charity Mermaids as a "devastating blow" to trans young people.