The SEATO zone boundary was 105 degrees East. We were already engaged in Malaysia
Indeed. With sometimes methods that were as if not more brutal than anything related to our meddling in Vietnam. I wrote a story about Brigand ops in the conflict (leather air brakes, are you serious?!).
But you won, so there's the difference.
My point was, the UK decided, for its own national interests, to not participate in Vietnam.
Which is how the game is played and to assume otherwise is even more whimsical than this thread.