I'd never do that on a 'normal roundabout, either, meaning the sort which takes 5-10s to traverse.
The idea behind that Highway Code rule is to support an allegedly safer practice when cycling (or horse riding) on very large roundabouts with two or more lanes all the way around; the sort found on major, dual carriageway intersections / motorway exits. The practice is for the rider (bike/horse) to stay on the 'outside' of the roundabout at all times to avoid motor vehicles 'undertaking' (which certainly feels uncomfortable, at least, to me in such circumstances).
The issue, as pointed out above, is that car drivers may not understand what the rider is doing and may just assume that the rider is leaving the roundabout at the next exit. Of course, the rider could signal right all the time, but that's unsafe / unstable in itself and could still be missed by the left-turning motor vehicle driver.
Net result: I'd not do this on a large roundabout either. Personally, if the roundabout is too alarming / busy / large to safely cycle around in the same way that a motor vehicle would, I'd get off and cross each of the exit roads on foot (or probably avoid that route in the first place if at all possible). If the roundabout is that big it'll often have pedestrian crossings. It'll take longer but greatly reduces the risk of death over the mounted options.
The practice enabled by the Highway Code rule makes a lot more sense for horse riders, but they have the advantage that drivers are likely to be much more wary around a horse on a big roundabout than they are of a cyclist.
