anyone know what this strange mark on wall means?

Yes it’s an ordnance mark. Originally in uk the army used surveyors to create accurate maps for distances and bombardment. That’s why our maps used to be called Ordnance Survey. That mark is the ordnance mark chiseled into a stone denoting a survey base marker for the local survey. Often the point would be taken from the top of the T shape where the arrow points to. Also the survey point might be the top or other chiseled mark on the object. These are typical in uk as they were added during heyday of surveying. If you had a local map and looked at it you will often see the ordnance points marked on the maps. These points denote usually a height above sea level that can be accurately used to continue surveying and are called ordnance datum points.