I was going to apologise, but you asked us not to. This could, I think, have been dealt with better, even if you were in breach of UK immigration law at the time, and while I can’t honestly say that I’ve heard that any other countries do much better at this (indeed, the USA has something of a reputation for awkwardness and I’ve heard plenty of tales, including from my wife, of your immigration folks waving handguns at “detainees” who in some cases were very likely in similar situations to yours).
I think, having read your story carefully and having taken a look at Smashing Media, the immigration officer may be correct that you were in breach of the law — had the conference been organised as a not-for-profit (rather than commercial) event, I think you’d have been able to enter under the Standard Visa terms, but it looks to me like it might have been intended to make a profit for Smashing. I know that seems unfair, particularly as we all know plenty of people who have turned up in the UK to do pretty much exactly the same thing and not been stopped.
If this kind of thing happens to anyone else, it’s very important to remain calm, and to ask the officers in question to clearly explain which rule they feel you have broken and why — it may well be that it is just a misunderstanding. It is also well worth contacting the event organisers to see if they have someone who can help you, and failing that to see if there is any free legal advice you can obtain (travel insurance often comes with it, as does household insurance, and some credit cards — especially American Express — offer assistance too). I’d like to think that had Rachel had legal advice at the right moment, she’d have been able to carry on to give her workshop, though I don’t know for certain that that’s true.
Of course, it’s also worth reflecting that really, the conference organisers, Smashing Media, should have checked in advance whether visas would be required (and if so, what type). If you’re inviting someone to speak or participate in your event, particularly if you make money from it, it really is your responsibility to get it right, and if that means paying a lawyer to tell you, then that’s what you should be doing.
p.s. If it cheers you up at all, you might consider what happened to a friend of mine, of Italian descent, whose father emigrated to Colombia to grow coffee. He flew from the UK to see his dad, via Canada as there weren’t any flights from the US to Colombia at the time. On entering Canada, he was greeted by a large, friendly man with a huge smile saying “Welcome to Canada… how long are you staying?” On explaining that his onward trip was to Colombia to see his Italian coffee-growing father, the smile dropped and he was sent to a back room for a cavity search, courtesy of a man with a very large rubber glove. He now narrates the story quite cheerfully and it always gets a laugh (after all, Colombian “coffee growers” from Italy are something of a stereotype, no?)