// ... hang on, they are doing their level best to prevent brexit ... //
They are doing their level best to prevent No Deal, at least until next year, and they have done their level best -- with the help of the ERG, ie the hardest of Hard Tory Brexiters -- to vote down the only agreed form of Brexit that has been reached so far.
I don't doubt that there are many who privately don't want Brexit at all that publicly say otherwise, and, yes, the Lib Dems are fundamentally against leaving, but in either case neither of them will countenance just straight up revoking the notification unless the British Public endorses that position in a future referendum and/or the next General Election.