I found two of these spiders under the floorboards of our wooden house. The shape was unlike anything iv seen before, like the shape of a black widow. These photos were taken last winter. The spiders were a little smaller than a house spider.
I had to use a torch beam to get the camera to focus on it.
This is a 'Cave spider', Meta sp., there are two UK species that are very similar and can only be distinguished by microscopic examination. Both species live in complete darkness, especially caves, so beneath floor boards would fit. They are harmless and do not usually venture out from their 'cave' environment.