I'm not sure how it works for transit. Also, if I were not a US citizen (and, even as a US citizen), I'd avoid the US as a transit country if at all possible -- lack of sterile transit makes it a pain due to waiting and scheduling even if you're a citizen, and if you're not visa waiver, it's horrible.
The UK does sterile transit, so most travelers shouldn't even have to touch security/immigration/customs if they're in transit only, which is what confuses me about this story.
The Heathrow airport is quite a mess, actually.
What one doesn't have to do is pass through border control or customs (unless one is transferring onto a domestic flight, then quite where you clear border control and customs varies from place to place — in the UK you clear the border at the port of entry and customs upon arrival at your final destination).
Also — just to note, I can't imagine anyone ever being actually arrested under the no-right-to-silence clause (and if they did, I expect it'd just get take to the ECHR and thrown out) except potentially for UK citizens/subjects/nationals/whatever-other-categories-there-are-that-I-always-forget (for whom entry cannot be refused).
If what you mean by not being sure how it works for transit is how it works in the US — you must posses a valid visa (probably a transit visa) or not need a visa (VWP, citizenship, etc.) for entry into the US, regardless of your further destination. Sadly, for many of us from Europe, many flights to South America require pre-approval from the US, because you cross US airspace, even though you never land.