If this is an Ordnance Survey trig pillar, there will be a brass plate near the bottom on one side. This will contain the letters OSBM and a number. The numbers are unique. If you tell us that number, we can identify the trig. To the best of my knowledge all live OS pillars have now been found. There is a sliver of a chance you may have stumbled upon a relocated pillar deemed 'lost or destroyed' to date.
The number will have either four or five digits.