Cool, that lamp really suits this!

Is it meant to run on a transformer ? Why the special plug ?

No, this will just be mains. The plug likely dates the fitting to a time when the UK still used 15A round-pin plugs, in the days before our current 13A versions. The plug you see here is a tough plug - there was a matching socket, also moulded in rubber. When plugged together, the combination also provided a degree of protection if used outdoors in a wet environment. They did offer a similar version in 13A years later.

Shows it's age a bit then, a great find

Now i just need the matching socket and wire that up to a standard plug to get this in action