Boris in London seems to also be expecting Biden to win.
The natural tendency is to go by the majority of polls.
British Prime Ministers don’t have a very good record on predicting American election results. Tony Blair famously went to bed in on election night 2004 convinced John Kerry had won. Bush won easily. In 2016, every single British political leader including Theresa May the Prime Minister was absolutely convinced Hilary Clinton would win, and totally ignored Trump. Theresa May had to make a quick trip to Washington to make up with Trump after the elections. They tend to believe that fake news media in Britain too much. You’d think they’d have learned something from Bexit.