The patch levels are not entirely consistent, one group of servers have an additional patch that the others do not, but this does not correlate with the timezone problem. Another server with the same patch level had a correct timezone file.

I read about the timezone changes, but they should not have affected the London file.

The only thing that stands out is that the working servers were all rebuilt to Vi3 last winter, and so would have been GMT at the time, whereas the ones with the problem were rebuilt this summer, during BST.