You also run a regenerator at 230v, so even if you are using a non-export 220v model, it's not so far out of spec. Without a regenerator the UK standard (240v +/- 10%) can see voltages as high as 264v, I don't fancy how a 220v transformer would behave in those circumstances. Actually it's more than a bit irresponsible to be implicitly telling people 220v is OK when for most, it isn't safe.