A partial or total eclipse is as dark as twilight, but only for a short period. Does it count as night flying for regulatory purposes?
I'm looking for a UK answer, but information about the US is interesting too.
No: that does not count as night flying.
The CAA defines "Night" (Air Navigation Order, Article 129) as:
‘Night’ means the time from half an hour after sunset until half an hour before sunrise (both times inclusive), sunset and sunrise being determined at surface level;
Similarly, the FAA's definition (FAR 1.1) is:
The time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the American Air Almanac, converted to local time.
Since an eclipse is neither a sunset or sunrise, nor does it happen between evenings and mornings, it does not fit the definition of "Night" for legal purposes.