Yes, our breaker/fuse panels are the same as your RCD consumer units, and yes, our electricity is a bit different. We run 120v on most household items, requiring twice the amps as you do over a single pole. To further, we require a hot and neutral wire. Because we don't have two fluctuating poles (on most circuits), the return doesn't go over a 2nd hot line; it is returned via a cold neutral instead.
A typical circuit has one hot, one insulated neutral and a bare ground. Within the breaker panel, the neutral is then essentially connected to ground. This *must* be done in the panel though. All neutral has to return ungrounded and unbroken right to the panel. You're not allowed to 'cheat' and just dump it into ground anywhere else in the system.
Typically, in a residence, only a couple of items run 220/240 and split the current across two phases... the stove and the clothes dryer. Normally like in the U.K., a 240v circuit (2-phase) doesn't require a neutral, but since the motor and any lights etc in a dryer still run at 120v, the 120 is pulled off of only one of the two hot poles, then returned through the neutral, exactly like in a wall outlet. The heating elements use the 240 2-phase, and does not use the neutral.
As far as OP, it is possible that the cable run is too long for the gauge of the wires, and what you have in the shed is trying too hard to pull electricity over that leg. You'd be able to test this by grabbing an "amp clamp" from your local hardware store.
In Canada, a book called "Electrical Code Simplified" can be bought at any decent hardware store. It explains the vast majority of what you'll experience in a residence in an easy-to-understand way. Worth picking up for anyone designing grow rooms.
...and one last thing. The 80% max circuit rule is calculated as such:
120v x 15A x 0.8(%) = 1440w
You should never run a circuit continuously past the 80% rule regardless of how far you are away from the panel.
-spek
