This definitely sounds like the classic odd man/woman out looking for a mate. Seeing the reflection of itself in the window makes the bird think that there is either a competing gendered swan or a possible opposite gendered mate. In any case, the swan should be someowhere else on the Thames or other natural water area. Please contact Wendy Hermon (Swan Lifeline) in Eton. She will be able to determine what to do in the best interest of this lone swan. It probably will be rescued, checked for any health problems and returned to an area where other swans (especially lone males and females) flock so that it can find a suitable mate. The Regal Swan