
The sentencing judge said Mark Colborne, 37, was a "warped individual" who was "consumed with rage at disparate individuals and groups."
He was convicted of preparing terrorist acts in September, after a jury had failed to reach a verdict in May.
Experts told the court they believed Colborne was "sane" but had a severe personality disorder that required treatment at a secure psychiatric unit.
Prosecutors had cited evidence from his diary in which he said he planned to use a sniper rifle to kill Prince Charles.
He also acquired chemicals, dust masks, funnels, syringes and gloves that he planned to use in a cyanide attack, prosecutors said.
Colborne denied the terrorism charges and told the court his diary notes about his plans were "angry rants" that he wrote while skipping his medication for depression.
The court heard that Colborne felt he was "marginalised" as a white, ginger-haired man. He had previously said he was an admirer of Norwegian mass-murderer Anders Breivik and planned to carry out an attack "for the Aryan people."
Speaking after Tuesday's sentence, Detective Sergeant Andy Hedley, an investigating police officer in the southern county of Hampshire, said Colborne was "a fantasist who had ideas and plans about committing acts that could cause great harm to people."
"Colborne went beyond the realms of fantasy when he wrote down his intentions and bought the chemicals that would have enabled him to carry out these plans," Hedley said.
He paid tribute to Colborne's family "who were brave in coming forward to tell us about concerns they had about his behaviour."
DPA