Michael Green, 48, told terrified workers he was going to "liquidise some assets" when he stormed into offices of the Advantage HGV training company on Tottenham Court Road, claiming he was laden down with explosives.
The former BNP candidate went on the rampage after twice failing to pass his HGV driving course with the firm.
When the company refused to refund his £1,000 fee, Green, dressed as a suicide bomber entered the office block and told staff he had nothing to live for.
After ordering staff to throw computers and paperwork out of the windows into the busy streets below, Green told a pregnant hostage that he was keeping her close by for "collateral damage".
Large parts of central London around Tottenham Court Road had to be sealed off while police marksmen and negotiators attended the scene on April 27.
Three tube stations had to be closed in the chaos and blood supplies to a nearby hospital were also affected.
Thousands of terrified office workers, together with many shoppers and tourists in the area, were evacuated during the stand-off.
At one point, Green told the general manager of the company: "This is a hostage situation I've got enough explosives to blow up the building. I've got nothing left to live for. I want you to get armed police here now."
Police marksmen, explosive experts and negotiators all attended the scene, while snipers took up position on nearby rooftops.
Appearing at Blackfriars Crown Court on Tuesday, Green pleaded guilty to charges of false imprisonment, communicating false information with intent, and criminal damage.
Jailing him for a total of six years Judge Henry Blacksell QC ordered he serve an extended period of three years on licence after his release for what he described as a "terrifying' incident" that had been prepared in a "quite chilling" way.
He said: 'What you did, and what you set about doing, and what you set about planning was a terrible thing and it demonstrates in my judgment a disturbing aspect of your behaviour."
The court heard that Green was a former customer of Advantage who bought a training course from the company with a view to obtaining a HGV license in 2011.
He contacted the firm by email and was offered a £1,000 in extra training but later failed the examination a second time.
In January 2012 Green had demanded a refund of his course fee but the firm said it would not give back his cash.
The court heard that Advantage HGV lost £165,000 as a result of the siege while London underground suffered a £53,000 loss.