The standard ES fitting has been around a long, long time, but for some reason Britain opted to use the double-contact BC (bayonet cap) type for most standard domestic bulbs.
The ES bulb is found here in some fittings, probably more so these days with the sale of the same fictures throughout Europe.
IEE Wiring Regs. specify that the live/hot shall be connected to the center contact.
As for the maximum rating, it depends upon the construction of the lampholder and the fitting in which it is located.
