Police and prosecutors said Joshua Kerry, 28, had been charged with engaging in conduct in preparation of terrorist acts between May 2024 and July 2026, saying these related in part to Farage.
They did not provide further details. Kerry, from Rotherham in northern England, is due to appear at London's Westminster Magistrates' Court on Wednesday.
"We have been running a hugely intensive and complex investigation for some time," Senior National Coordinator for Counter Terrorism Policing Vicki Evans said in a statement on Friday.
"The charge announced today is a result of the tireless work of investigators across the counter terrorism policing network."
Kerry is already facing accusations of murdering Widdecombe, 78, a prominent figure in Reform UK, who was found dead at her home in rural southwest England on July 9 after being attacked the day before.
The killing of Widdecombe, who prosecutors said was struck 21 times with a hammer, shocked British politics, raising further concerns about politicians' security, with two serving British members of parliament having been killed since 2016.
Farage, 62, often credited with forcing the 2016 referendum on whether Britain should stay in the European Union, has been one of the most influential, but also one of the most divisive, figures in British politics for more than a decade.
When campaigning for parliament in 2024, Farage had a milkshake thrown at him by a woman and on another occasion objects were hurled towards his open-top bus by a man.
Reform UK was leading in the polls until recently, though a combination of allegations over party funding and Andy Burnham taking over as prime minister has seen an erosion in its support.
Farage is currently facing a parliamentary investigation over whether he should have declared a £5 million ($A9.71 million) gift from a billionaire cryptocurrency investor.
He has since said the money was necessary to provide him with security.
