“Trade deals between complex economies take time - and the USA and UK have a lot of differences in opinion.”
So did the Mexicans and the Canadians. President Trump managed to nail that down in a few months. It won’t be any more difficult to work with the Brits.
It wasn't a few months (and that's not Trump's fault in any way) -- the formal negotiations started on May 17th 2017 and was not concluded until September 30th 2018 -- 16 months later. It's still not ratified by all three countries
A deal with the UK would take 9 to 12 months to negotiate and then longer to get ratification through both legislatures before accepted by both countries.
trade deals take time. The UK would be trading with the US and EU under WTO rules until that happens. That would impose tariffs on imports and eliminate most of the J-I-T manufacturing done by the UK as part of the western european supply chain
The UK will get a US trade deal post Brexit, just don't expect it before the end of this year or more likely next year.