Don't take my word for it: read Stephen Bush in the New Statesman:
One thing is clear: the "Liberal Democrat fightback" is not just a hashtag. The party has been doing particularly well in affluent Conservative areas that voted to stay in the European Union. (It's worth noting that one seat that very much fits that profile is Theresa May's own stomping ground of Maidenhead.)
It means that if, as looks likely, Zac Goldsmith triggers a by-election over Heathrow, the Liberal Democrats will consider themselves favourites if they can find a top-tier candidate with decent local connections.
They also start with their by-election machine having done very well indeed out of what you might call its “open beta” in Witney. The county council elections next year, too, should be low hanging fruit for [them].