You may also see little plaques on walls, sides of buildings etc. These have a logo like an upward pointing arrow with a dash - on the top....a bit like a side view of a picnic bench. It will say a figure such as 102.3 AOD. This means the height of that little dash above sea level (Above Ordnance Datum). These are scattered all over the UK as part of a network to assist surveyors.
Your local council planning dept. should have GIS (Geographical Information System) and theoretically could pull up an electronic map of your property and it will give the height above sea level.