David Cameron and I visited the Open University the other day, he to give a speech to the world, me to learn something about day-old chicks and biochemistry. Neither of us knew the other was going to be there. I was told at the reception desk to wait on one of the seats behind me and handed a label, one of those clip things I can never work out how to fix to myself. As I turned to go to the chairs, preoccupied with my label dyspraxia, someone grabbed me by my elbow and pulled me to the left. I dislike being grabbed so I pulled away and carried on the way I was going. But I'd failed to notice that the world had changed while I had my back to the room, and a semicircle of large chests in suits were claiming all the space for the man at their centre. Walking towards me, as I was towards him, David Cameron, slick as an oil spill, tailored to within an inch of all our lives, made his way to the reception desk to sign the visitors' book.

The danger of collision only lasted a second or two before I swerved left to one of the seats, but as I walked directly in his path his face performed an expression – lit up, I'd say, except that even with what looked like make up, David Cameron's doughy face is not designed to emit light. The smile came all of a piece, public smile number 3, the one you use when a voter is coming towards you and you're not sure if there's going to be confrontation or congratulation, but you have to keep the look right because the fucking cameras are clicking. Actually, I was just going where I was going, and hadn't twigged that I'd landed in a Special Moment.

It appears that public figures like David Cameron have to walk in straight lines, from point A to point B, unlike regular folk who expect other people to be using the same world and learn to accommodate. Even penguins – which do have certain Cameron-like qualities – can manage it: if you stand in their way, they stop dead in front of you, conclude that you are a novel sort of rock, and then make an impatient detour around you to return to their chosen path. Unlike the penguins, Mr Cameron had his minders to make sure nothing got in the way of his straight-line progress.

In the event, we didn't collide, I found a chair and waited for my friend, while David (I feel I can call him that now) continued his uninterrupted route to the waiting visitors' book. But the thrum and zing of importance, of hot lights, of air-time past and future, clogged the atmosphere: a VIP was visiting and I now saw that everyone, the receptionists, his hosts, even his minders, were rigid with excitement and reverence. Strange, when you come to think of it, because the Open University is stuffed with illustrious, serious thinkers and educators, not least the biochemist I was going to visit, who finally arrived to take me to his laboratory. 'I've just bumped into David Cameron,' I told him. 'Oh,' he said.