"The LibDems campaigned in the referendum, then when it turned out they'd lost, they decided they weren't going to abide by the result, they were going to arbitrarily cancel 17.4 million votes and overturn it."

How on earth could the Lib Dems arbitrarily cancel Brexit??

The only way would have been to try to convince a majority of the country and for the country to decide by a GE or a referendum.
And that is how our democracy works.

"And if you still don't think the LibDems are now extremists, can you tell me of any policy, promulgated by any mainstream political party in the UK, or indeed in any democratic country, that was more extreme than overturning the result of the largest democratic vote ever held in that country ? In your own time mon ami."

Hanging was an issue that divided the country. It was finally abolished in 1969 after many years of argy bargy from both sides.
There is a move by some Tory MP's to bring hanging back. If there were to be a referendum on it, and it was passed, then I for one (and I suspect Labour and the Lib Dems) would campaign strongly for it's abolition once again.
But we would not be "going to arbitrarily cancel 17.4 million votes and overturn it." That is complete nonsense and you know it.
But we would strongly campaign for it's abolition - which has been normal practice in a democracy since the year dot.

The suffragettes spent years campaigning to get their wish. Are you calling then fascists?