Re: Bah!
If there's such a thing as a low-current UK extension cord, it's liable to have a current fuse in the plug that pops if it's overloaded. I once made an extension cord with cable rated 5A, since I happened to have that cable I suppose, and fitted a 5A fuse. We had a slightly amusing incident one cold Christmas when I put an electric heater on it, using 1 kW, which was fine. Until my sister turned the heater up to 3 kW, as I said "No!" loudly but too late; of course the fuse popped. I think I found a spare fuse, or another plug to take one from...
I did similar to myself with a multi-socket extension, which I think had the intended feature that three sockets would only turn on when the fourth was supplying current, so that you could turn on, say, your TV, and the rest of your "home theatre" stuff also switches on. This did have a limit of 1kW, I think, for no clear reason, and a tricky internal fuse, but I was short of an extension and I assumed that it would cope with a 700W domestic microwave oven. It didn't, because such a microwave produces 700W of microwave energy but uses somewhere above 1000W to do it, as it's less than 100 percent efficient. Statements online include "50 percent" and "650W / 1000W". (And there's a motor for the turntable, but that can't be much.) The actual number is in the machine's small print. So again the special fuse popped.