Ok I got two of those VGA cables off eBay for £10, and can confirm they did not work. Furthermore, when I tested them, the power light on the Jaguar started flashing, and then it went off completely. Luckily, there doesn't seem to be any damage to the console, as when the controller without the extension was returned it booted-up fine again. Rather than risk frying my Jaguar, I think I'll just sit closer to the TV, until such time as when dedicated extension cables are available in the UK.
(I have used a Team Tap as a little extension cable before, but it's not ideal).