This is a real long shot, and I’ll be amazed if the Dope can come up with an answer, but then the Dope has amazed me plenty of times before!
I dimly recall reading a book when I was a kid, in probably the late 1980s. It might have been a short story in a collection rather than a standalone novel. The plot revolved around some kind of time-slip to (I think) WWII-era London, from the then-present day, say mid-1980s. I also seem to remember the Thames Barrier, which was new at the time, featuring.
I think the main character was a young, maybe teenage, boy, and I’m pretty sure it was a children’s/teenagers’ book.
And for some reason I remember the main character repeating like a mantra “Today is Tuesday February 12, 1986” (or whatever the “real” date was) in an attempt to somehow hang onto reality when the time-slip happened.
Does this ring any bells?