What surprised me a little, and prompted the question, is that with one exception (traffic crossing 1000 ft below) there was no one visually or on the traffic system showing within 6-8 miles anywhere close to my altitude for the whole time I was on headings. (I don't have full TCAS, but generally would expect to detect large aircraft transponders quite well.) It was from this that I guessed there must be some procedural rule or mental advantage London Controllers have when putting someone on headings. It doesn't seem to happen anywhere else and certainly my US, Dutch and German experience is that getting a heading means you are consciously being vectored off ones flight path rather than just ensuring you remain on the flight path.