...there was also War Plan Red, and yet America and Britain got along just fine at this time, and afterwards.
My point is, for all the the USA was preparing contingencies, they weren't default hostile to Japanese goals until Japan acted on them with the 21 Demands.
No...
The US built for Scarlet, too. With people like King, later, it is easy to see that relations between the US and the UK were never "good" the way popular historians make the populaces believe. There were hot-button issues like India, British exclusion of American goods from British controlled territories, clashes over oil interests, some nasty stuff involving Venezuela, etc. And the issue over who would rule the oceans. The USN was hellbent on ending the Royal Navy as the top navy, one way or another. Guess what happened under Roosevelt? (the other one.)
In modern terms it is almost like the PRC and the USA today. Rising power scraping elbows with a decliner. It could go UGLY very quickly.