By the way, this goes without saying, but there's more fine print further below in the link (which I assume is satisfied in the scenario outlined by the poster):

"Do I qualify for transit without visa (TWOV)?

If you are a visa national or a DATV national who is exempt, you can transit without visa if you will:

arrive and leave by air within 24 hours and have no intention of staying in the UK (you can travel by rail or road between two airports); and have a confirmed onward booking that will leave within 24 hours of your arrival in the UK."

Precisely! It does sound like I do not need a transit visa and I went soo close to purchasing that itinerary I mentioned above (thankfully I didn't). Then again, the answer when I called (on phone) the British embassy is that I do need a transit visa. That answer was provided after the agent/officer heard about my detailed travel arrangements and the documentations I hold onto. I have a four year long term visa in Canada and a short term visa for Singapore.

There could of course be a chance that, that particular immigration officer was wrong at this (as 111op mentioned). But I don't want to take any chances, and am going to take the pacific route at least for this time! THANK YOU SO MUCH FOR ALL THE REPLIES.

Well, maybe they need to hire better educated consular officers.

I was going to say that it could be because this particular exemption is new, but my hunch is that it's been around for a few years, actually.

Anyway, for a future visit, e-mail them and get something in writing. In fact, e-mail them now while you remember this (since they can take a week to reply) so that the next time you'll know for sure.

Plus you can always print out the regulations. I don't think that anyone can dispute something written in B & W published on their own website.