Or some other event a lot of people might want to go to - a big-name rock concert or something?
I have a backup plan involving the Scottish National Referendum but sports etc. would work better.
Searches tried - European football 1997, Sporting events September 1997, Current events September 1997, etc.
Later - sorry, while this was going through the moderation queue I've had to change some plans for the story - this has to occur in the week or so before September 15th 1997. To clarify this a little, I'm looking for a reason for a lot of people (say a bus full) to travel into Britain from France on or before this date, so that some fugitives can slip into the country with them. Returning from a sports event or something like a rock concert, or travelling to one, seems like a good way to explain it.