There are multiple breaches of EU. treaties here Frenchie which state catagorically that EU citizens living in other EU countires have the right to vote in these elections under exactly the same conditions as the native population. Contravention of these treaties could well see the British votes regarded as null and void, and Britain being asked to vote again. Just one humiliation after the other for this country I'm afraid.
You are right cologne, there are multiple breaches of the law for the reasons you give. It seems unlikely that the election could be held again, but the government could find itself in court yet again. What we are seeing is the "hostile environment" encroaching on democracy now.
Sorry I missed the punchline.