The same thing recently happened to Jorge Cham, the cartoonist who draws PhD Comics, when he tried to enter the UK for a lecture tour. A cartoon describing his experience is here: http://www.phdcomics.com/comics.php (Oct. 26 comic).
If I were visiting the UK in the near future to meet with colleagues or give a talk or etc., I would be sorely tempted to simply tell the border agent that I was visiting as a tourist and would not be doing any work whatsoever, volunteer or otherwise.
