This is my first post on this site!!
Yes this is a really good project but for the UK controlling a light inside of a wall switch you would need to do the following.
Re-wire the ceiling rose as in the UK there is a live feed from the rose that is switched by the switch and then returns to the rose to the "switched live" See http://www.diydoctor.org.uk/project_images/lights_and_switches/ceilingroseb2.jpg
Put an extra cable up to the ceiling rose with the return cable from the board that goes to the switched live and add an extra terminal in the ceiling rose to link the neutral to the other side of the lamp fitting.
If the back box is metal, it would need to be changed to a plastic box , I wouldn't myself want to mount the board in an earthed metal box!!
As other contributors have noted there isn't any fusing on the live and neutral feeds if the SSR were to fail short because of a short circuit in the lamp or ceiling rose then there could be a large current going through board that could potentially cause a fire, safety is always the first thought with any mains electrical circuit.
I could see this board being used in a plastic box inline to a table lamp etc. but it would need a lot of work to re-wire your house if you intended to use it behind a wall switch.