How to use both British and American English?

You can filter mu_dropdown_languages and re-add American English here.

Sample code, not tested:

add_filter( 'mu_dropdown_languages', function( $output, $lang_files, $current )
{
    array_unshift(
        $output,
        '<option value=""' . selected( $current, 'en_US', false ) . '>'
            . __( 'American English' ) . "</option>"
    );
    return $output;
}, 10, 3 );

Filter sanitize_option_WPLANG or pre_update_option_WPLANG to jump into the save process and correct any possible issues.