It's very easy to dismiss this as a comic event, but I don't know what I might have done, or felt, if I had been in a similar situation. I can imagine that the "perpetrator" might have been acting suspiciously, maybe a little nervous having a sly "smoke" on a bus furtively looking around in case anyone sees him.
But would I think it likely that a terrorist would want to bomb a lowly Megabus instead of a high profile train or plane? It might seem ludicrous - but think about it. A bomb on a bus might kill 40 - 50 people, which is headline grabbing enough, but if it was coordinated with simultaneous events on the M6, the M1, the M4 and M25 it would seriously damage our transport infrastructure and could distract the emergency services long enough for the "real" event somewhere else.
The fact that someone reported it should be applauded, as should the reaction of the emergency services. On the information available at the time it could certainly have been real and therefore should have been taken seriously. If we mock it, and the reaction, it may well prevent others reporting something that could be really dangerous. Hell, who knows, the whole thing may have even been a test to see how the services did react. I think we did rather well, actually.