I can't see signs of recent leakage, at least since it was painted. Bare copper shows green or white marks around leaks.
It looks to me like the pipework is soldered copper. A tiny leak here could be filled with limescale. This would be a replacement or additional pipe since the house was built. It's the mains supply pipe so would be at high pressure and a tiny hole could spray out water.
The water damage seems to be around the pipe, not the stopcock.
It looks to me like cold air would blow up the hole in the floorboards, so I wouldn't expect condensation. That is more common where a cold pipe enters a warm, steamy room, especially a bathroom. I suppose it's possible builders hid rubble under the floor and blocked ventilation.
The floor looks like softwood floorboards, which are not easily damaged by occasional damp.
If you see any future signs of damp, including woodlice (which eat damp and rotting wood) then the suspect pipework would be cut out and replaced with new without much difficulty. At this point I'd replace any timber showing signs of rot or long-tern damp.
Try not to disturb it, because movement may open up any bad jointing. If you rub it down for repainting this might start a leak. There are leak sealants which give a temporary bodge.
If you do any work on it, you might consider running it under the floor where it will be out of sight. You can lag it under the floor with Climaflex or similar. If you go for a Combi or an unvented cylinder you would benefit from replacing it with larger plastic pipe.