Identifying valve in a flat
If the valve handle is any indication for what its' function may be, than there is a very likely chance that it is a shut-off valve for: A. water supply (for flat above?) or B. sprinkler system (not likely).   The valve type is either a "gate" or a "globe". Either of them are designed to control water (liquid or vapor) flow. Additionally both valve's have a poor reputation in that the valve stem can break off from the gate or globe (the section that closes the opening to stop flow). It would be wise to first ascertain what the valve is controlling before it is opened or closed.