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@vfrmedia OK, my best guess is that it's either Dytiscus marginalis (great diving beetle) or some not-too-distant relative.

@vfrmedia (This is from image-searching for `"england" "water bug"`, then discovering that water beetle got more plausible results, and finally finding that some were called diving beetles -- that's what gave the best matches. And then finally some of those had similar markings.)