Andrew Tweedy, 30, was sentenced Friday, the Daily Mail reported.
Newcastle Crown Court found that, although not present at the May 10 heist, he plotted it and loaned the bandits his fake Beretta handgun.
The gang drove from Tweedy's home to the post office, returned there with 2,000 pounds ($3,200) in loot, and hid the gun and a machete in his attic.
Police found the getaway car parked behind Tweedy's house. Within days he was arrested, confessed and named his accomplices.
Tweedy, who has 90 convictions dating from age 13 and a longtime drug problem, pleaded guilty to conspiracy and weapons charges. His partner, Emma Stanners, with whom he has a child, was cleared of money laundering.
Robbers Christopher Swindle, Christopher Finlison, Craig Glendinning and Jamie Percival got sentences ranging from six to seven years.
Cole, 27, is a member of the girl group Girls Aloud, a judge on the "X Factor" TV talent show and a model.