There is another political theory for its cancellation.
There is a tale of a certain Brit PM being 'got at' while on a visit to Russia.
After this visit many subtle but long lasting changes occurred in the UK, and the unions gained in power. The Americans clearly didn't want competition, but then nor did the Warsaw Pact.
This story does come from credible sources IMHO, but could not be proven either way as no one would gain from admitting any of it.
If the cancellation was not purely on rising cost, or the 'fixed price Concorde data research contract offer' after cancellation, wasn't down to U.S. Influence over the Brit loan from the IMF, and couldn't be put down to there being no need for manned bombers, then it has to be something else.
I'm no conspiracy theorist, and am happy to believe it was due to a group of people with massively inflated egos and a over developed sense of self importance (politicians) who just didn't like it or wanted to do something to boast about later.