Have most european mainland countries switched to a genuine 230V, or are they still running at 220V? I suspect the latter, because 220V is within the range of 230V+-5%, just as 240V just on the opposite end of the scale. By my understanding any UK supply running at more the 241.5V is exceeding that percentage, although I've no idea if the tolerance they actually specified was 5% or 10%, and I don't know anyone who'd complain. Even transformer based supplies which aren't as tolerant as switched mode supplies to different voltages would only actually dissipate 1.2V at whatever current across the voltage regulator for a 12V DC supply running on 240V versus a designed for 220V. Although in my experience 12V supplies are more commonly unregulated because a motor or whatever you're driving with it itself doesn't care a jot whether it actually gets 12V or 13.2V. For a 5V supply it only has to dissipate an extra 0.5*current watts of heat.