Yea, honestly that seems a way higher priority than "Macarthur's focus tree." The French AI leaves its borders undefended because its men die in Baltimore.
I mean you're free to handwave this however you want, it's just something I find plausible given America's actual expansion west and Britain's foreign policy.
British troops left for Canada in February, so they will be showing up in short order anyway. Why would they sail away without testing the defenses? And in the interim the US occupation will be... maybe not opposed, but hardly welcomed.