Many Brits live under the false impression that the UK has a Special Relationship with the US. In reality, it's far more transactional, especially for the Americans. This delusion is especially dangerous as post-Brexit UK seeks a new place in the world
While the two do have a very close relationship on security and defence built on post-war efforts, it would be foolhardy to expect any US president to place much emphasis on the UK's economic interests over any other third country
The problem Boris Johnson has is that a close relationship with the White House is more important than any time since the war, given the UK's new status outside the EU. What a headache, then, that the US is having an election with 2 v different candidates
On one hand: Trump likes Brexit, dislikes the EU and might see benefit in disrupting the EU by teaming up with the UK in a way that helps the UK economically. However, Trump is wildly unpopular in the UK and Boris does not want to be seen as his best mate
On the other: Brits are generally more pro-Democrat & more pro-America when a Dem is in the White House. However, Biden clearly does not support the UK's Brexit position, especially when it comes to Northern Ireland.