We can't be friends with both America and china because of Taiwan. Right now we're with America, the entire intelligence service is wrapped up together with five eyes and military cooperation through NATO.
Cosying up to China means giving all that up, so what's the point?
He's pro Trump and his administration was pro division so supporting a similar nationalistic movement as leaving the UK was easy, no the UK is struggling to keep the lights on he can ride the ass he meets