Q: Context: In the UK, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921, the datum was MSL at the Victoria Dock, Liverpool.

Question: where is sea level measured in the uk?
Let's give stream of consciousness below
The relevant sentence in the passage is: In the UK, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. So the answer is Newlyn in Cornwall between 1915 and 1921.