What are these things on my window frames?
I removed one of these devices, and behind it I found two voids in the frame leading to some more foam. I believe that these are "trickle vents", which are required in UK building regulations. In a modern, well insulated house, there is little exchange of air with the outside, which can cause moisture to build up in the air leading to condensation and damp. Trickle vents are intended to prevent this from happening. The protruding thing in the middle of the vent pictured allows the vent to be sealed shut.