Definitely cheaper to make your own at UK prices. Get the copper online though; plumbers' merchants tend to be considerably more expensive.
A useful bonus with one of these is that you can use the hot (scalding at first - take care) water for cleaning equipment. Probably not a huge saving in fuel and water, but better than just dumping it straight down the drain.
BTW, there's no real point in using ice-baths, etc unless your supply of water is severely limited. Mains water is quite cold enough if the flow is reasonable. It probably wouldn't be difficult to work out the optimum exit temperature from the coil and adjust the flow for that, but I simply set it to just attain the max. temperature possible, and that seems good enough. You don't have to watch over it all the time.