It was no surprise that photographers were chasing her; what may have surprised them, however, was to find themselves snapping an international celebrity, not at Heathrow but inside a railway station.
Miss Christensen chose to travel to London by Eurostar train, via the Channel tunnel and disembarking at Waterloo station, which must have been her first encounter with the great British commuting public.
She is far from being the only celebrity user of Eurostar's London to Paris or Brussels service. Last Friday afternoon at Waterloo, a film crew from the French TV station TF1 was trailing the actress Jane Birkin, and in the mad dash between fashion shows in London and Paris the week before, the Chanel-suit-and-heels count rose noticeably.
Politicians have been commuting frequently to Brussels, Eric Cantona was spotted on it after one of his recent court appearances; actresses, publishers and writers have all taken their places on board.
Despite the much-publicised false starts to the service, the volume of two-way traffic in and out of Paris has risen dramatically. There are those who go so far as to say that the British are now infiltrating Paris, though the Paris tourist board doesn't quite buy that.
Given that Eurostar reckons to increase its Paris services by the summer from three trains a day to a train every hour, each with a capacity for 794 seats, it clearly expects a market of Brits who hitherto would not have dreamt of whizzing off to Paris for the weekend.
Certainly a trip through the tunnel is becoming a rite of passage, something it is essential to have done or plan to do. The recent experiences of a colleague and his partner in Paris for the weekend are indicative of this growing trend.
They planned Sunday breakfast at a caf in St Germain, described in a guidebook as the haunt of local intellectuals. They left their hotel with a tweedy Brit practising his golf swing and bumped into another scurrying back with the Sunday papers under his arm. They arrived at the caf to be hailed by a former colleague, who was there with his family. By the time they left, the caf had a quorum of fully paid-up liberal Brits, if not exactly intellectuals, with Peter Preston, editor-in-chief of the Guardian and Observer, ensconced in a corner with his paper.
Even their daughter did not escape her British friends. On the train back to London on Sunday evening she found one of her classmates only two rows away.
Part of this rush is, predictably, driven by novelty. About a quarter of the British crowd getting on the Eurostar at Waterloo last Wednesday lunchtime would not have been going to Paris at all had it not been for the "try-out-the-train" factor.
Jean Quick from Devon was with her husband Anthony, a retired headmaster; their son had given them a couple of return tickets for Christmas. "Actually," explained Anthony, "it isn't the easiest way for us to get to France, because we live in Devon. Flying from Exeter or Bristol would have been quicker."
"I didn't want to go to Brussels," said one man, when asked why he was going to Paris, betraying that he had clearly limited his holiday options to one of the two Eurostar adventures.
"It's the thrill of going so fast under the sea, going direct from London and avoiding all the hassle at the airport," explained Mrs Houstons, 72, from Guildford. She would not, she explained, normally pop over just for a shopping spree. This was a one-off, "for fun".
Inevitably, those acclimatising most easily to the idea of Paris as a metropolis as accessible from London as, say, Birmingham, Manchester or Liverpool, are the British businessmen.
For Bob Earl, director of a housing corporation, the city is suddenly appealing to his imagination as a possible venue for a staff training ground. "I sometimes took them to Calais for the day on a management course," he said, "so why not Paris now?"
Still, there were one or two non-business adventurers in the departure lounge showing the first signs of broadening their horizons. And then, of course, there were the inevitable anoraks, the train-spotters who can time a train by every bend and minor station it flashes through.
The trip is a mixture of the humdrum and the dramatic. For what seems like hours, the Eurostar trundles along British tracks, going no faster than the average commuter train, its passengers on seats which in standard class are no more comfortable than utilitarian.
The announcement that the train is about to enter the tunnel breeds anticipation and then excitement as the train enters. Thereafter it is just black, dark and rather boring.
The emergence on the French side is dramatic, not least because the train starts to travel at the high speeds it was designed for. But what it lacks as a train journey it might make up for in profundity as a cultural experience: some have even predicted that it will change the nation's consciousness and make us feel more European.