In the end I went with jeans, green, not traditional blue denim, and wore a smart-ish top and accessorising scarf, so it was a sort of compromise between usual work formality and weekend clothes. Needn't have worried, as almost everyone else (including my boss) was wearing jeans anyway.
We were at the Royal Academy of Engineering for the day, which is a beautiful building off The Mall, which we could see out of the windows. I should think the Queen likes the view from her place, it is very attractive, and there are always loads of people about to watch.
The flagpoles with crowns and tassels on top were particularly appealing. I might get one for my front garden. Get a load of different flags for when various friends and family come to stay.
I decided not to stick around for the drinks afterwards, and by scampering back across the bridge to Waterloo at high speed, caught an early train and was home before 7.30. Result.
And now it's the weekend. Hurrah.