Electrician here.
@Marky L's bodge isn't quite as unsafe as you would imagine. The top pin on a plug is used as a path for current to flow to earth, should this be necessary (when there is a fault). As a safety measure, British three pin plugs will not enter a socket without the top pin present. You may notice that the top pin is slightly longer than the bottom two pins; when the top pin enters the socket, the guards over the socket holes for the bottom two pins will open and allow the bottom two pins to enter. You can't access the bottom two socket holes without the top pin entering first. This is to stop kids, or anyone, sticking their fingers in a socket and getting a shock.
Marky's pedal board power supply is a 'class 2' bit of kit. Class 2 kit doesn't use the earth pin for safety (it doubly insulates the live terminals inside the plug instead). Consequently, he only needs the top pin of the plug to be there to open the bottom two pins and connect the plug to the power source. The big lump of a pedal board PSU plug houses a transformer, which takes the 230 volt mains down to a pedal board friendly 9v (or something similar). 9 volts are quite safe.