No, no, Britain wasn't preparing for war against the USA. Plan Red was the USA's preparation for war against Britain.
And war plan Orange was for fighting a two front war with Japan and Great Britain.
In regards to Britain, from the book "
Thinking the Unthinkable: British and American Naval Strategies for an Anglo-American War, 1918-1931" by Christopher Bell:
"The British military never prepared a formal plan for war with the United States during the first half of the 20th century. For instance, the government of David Lloyd George in 1919 restricted the Royal Navy from building more ships to compete with American naval growth and thereby preventing the plan's development. Like their American counterparts, most British military officers viewed cooperation with the United States as the best way to maintain world peace due to the shared culture, language, and goals, although they feared that attempts to regulate trade during a war with another nation might force a war with America.
The British military generally believed that if war did occur, they could transport troops to Canada if asked, but nonetheless saw it as impossible to defend Canada against the much larger United States, so did not plan to render aid, as Canada's loss would not be fatal to Britain. A full invasion of the United States was unrealistic and a naval blockade would be too slow. The Royal Navy could not use a defensive strategy of waiting for the American fleet to cross the Atlantic because Imperial trade would be left too vulnerable. Royal Navy officers believed that Britain was vulnerable to a supply blockade and that if a larger American fleet appeared near the British Isles, the Isles may quickly surrender. The officers planned to, instead, attack the American fleet from a Western Hemisphere base, likely Bermuda, while other ships based in Canada and the West Indies would attack American shipping and protect Imperial trade. The British would also bombard coastal bases and make small amphibious assaults. India and Australia would help capture Manila to prevent American attacks on British trade in Asia and perhaps a conquest of Hong Kong. The officers hoped that such acts would result in a stalemate making continued war unpopular in the United States, followed by a negotiated peace.
Canadian military officer Lieutenant Colonel James "Buster" Sutherland Brown developed an earlier counterpart to War Plan Red, Defence Scheme No. 1, on April 12, 1921. Maintaining that the best defense was a good offense, "Buster" Brown planned for rapid deployment of flying columns to occupy Seattle, Great Falls, Minneapolis, and Albany. With little hope of holding the objectives, the actual idea was to divert American troops to the flanks and away from Canada, hopefully long enough for British and Commonwealth allies to arrive with reinforcements. Defense Scheme No. 1 was terminated by Chief of the General Staff Andrew McNaughton in 1928, two years before the approval of War Plan Red."