I don't think the stated ratings on those plugs has anything to do with their load carring capacity. They were designed to be physically incompatable with eachother to differenciate low power and high power circuits. They seem to be all identical in all aspects just miniturised in the case of the 2 amp and over sized in the case of the 15 amp.

My guess is that they just picked nominal ampages and assigned a particular size of plug to each with a good margin of safety rather than actually designing the the neatest possible plug to carry a X amps.

I'd say if they were actually rated for real load carring capability the 5amp BS 546 could safely carry 16 amps and the 2amp would be more like the modern europlug while the 15 amp would probabally safely carry a lot more than 15/16 amps given the enormous size and contact area on its pins. They're bigger than a lot of modern industrial connectors and it makes a pretty good connection considering that you can hardly get the damn things out again ! [Linked Image]