But what particularly frightened Allen, she says, was a reference he made about buying a rope. "It was absolutely terrifying," Allen recalls. "I was really, really, really scared."

"You shouldn't have to live like that," Allen says, "but, it's normal now."

This is a reality not just for Allen, but many other British lawmakers, who are receiving an unprecedented number of threatening messages since the UK's vote to leave the European Union in 2016.

Cox was the first British lawmaker to be killed in office since a Conservative MP was killed by an IRA car bomb in 1990.

Police statistics showed crimes committed against MPs had more than doubled from 151 in 2017 to 341 in 2018.

"A lot of those threats are very real," Alice Lilly, a senior researcher for British think tank the Institute for Government, told CNN. "We have seen several people prosecuted for sending threats, we've seen somebody prosecuted for plotting to murder a serving MP -- so these are very substantial threats."

Females and MPs from minority backgrounds 'disproportionately targeted'

When Brabin was elected as MP for Batley and Spen in 2016, she says she made a conscious decision to ensure she was more accessible to her constituents than any other MP.

"The two women who were with Jo when she was murdered became my staff members," Brabin explains, "and I said to them if we hide away -- if I can't walk to get a sandwich, if I can't get a newspaper without protection, if I can't go to a lunch club without having someone with me, we've failed."

"My family were concerned," she admits, "but my real belief is lightning doesn't strike twice, but if I hide away they win. If it looks like it's affected me, it's a magnet for more because you're the victim."

While Brabin says she received "very little abuse" when she first became an MP, she has noticed an increase since UK Prime Minister Boris Johnson started referring to the Benn Act as the "surrender bill."

That legislation, which was passed as law by Parliament on September 9, aims to prevent the UK from crashing out of the European Union without a deal and requires Johnson to ask for a delay if no Brexit deal is agreed with the EU by October 19.

"In the last few weeks it has escalated and we've had for the first time threatening phone calls, threats to push me down the stairs, to drive me over with a car... But I'm not afraid. I'm not taking it too seriously," Brabin said.

In fact, Brabin says the frustrating thing is the hours her staff spend sifting through "vile abuse" on her social media channels -- 75% of which she says comes from people outside of her constituency.

"It's anti-democratic because it stops good local people from engaging with me. All you get is one voice, and one tone, coming from all over the country."

Boris Johnson's rhetoric is 'fueling hatred'

With words like "surrender," "enemy," and "betray," Johnson recently has come under fire by MPs for spurring on hatred within the UK.

In a statement to CNN, a spokesperson from Downing Street said: "The PM has been clear that threats and abuse against MPs are totally unacceptable and that no MP or anyone else in public life should face threats or intimidation."

"Many of us in this place are subject to death threats and abuse every single day. Let me tell the Prime Minister that they often quote his words -- surrender act, betrayal, traitor -- and I, for one, am sick of it," Labour's Paula Sherriff said in Parliament to Johnson, who then dismissed it as "humbug."

Labour's Jess Phillips, who has spoken frequently about the abuse she has suffered as an MP and has admitted to carrying a panic alarm with her, also criticized Johnson for his language by tweeting a photo of a threatening letter she received at her office.

"This week I received an anonymous letter to my constituency office," Phillips tweeted. "@10DowningStreet might think we are "humbugs" about his words but they are literally being used in death threats against me."

The letter referenced the phrase "I would rather be found dead in a ditch" that Johnson used when he was asked if he would extend the Brexit deadline. The anonymous writer warned: "This is what will happen to those who do not deliver Brexit."

Johnson later defended the language he used in Parliament, but apologized if lawmakers had misunderstood his use of the word "humbug" in response to their concerns.

"My use of the word humbug was in context of people trying to prevent me -- us -- from using the word 'surrender," he told the BBC. "I can certainly say sorry for the misunderstanding."

Brabin told CNN, however, that Johnson's language was helping "fuel hatred" and because anxiety is high among constituents, "the voice of panic and fear is heard louder."

"The Prime Minister has an incredibly loud voice with a massive reach and he leads the way -- his tone filters down," she says, adding that as a result "the misleading comments from Number 10 are picked up and regurgitated."

While Allen agrees that the abuse MPs receive has escalated since the Brexit vote, she says social media is the main driver behind the increase -- especially as people believe they can hide under the cover of anonymity.

"I don't think it's just Brexit," Allen said. "Social media gives fast, hidden, easy access to public figures in a way that I suspect didn't exist for MPs in the past, and you can be anonymous -- or you think you can."