I was curious and googled a bit. Some British people on netmums.com report that they successfully put these names on birth certificates: Éilis, Zoë. I would gue

I was curious and googled a bit. Some British people on netmums.com report that they successfully put these names on birth certificates: Éilis, Zoë. I would guess that "Michèle" would also be allowed. I've never heard of an official set of allowed characters, but everything relating to names of people seems to be unregulated in the UK. One argument people mentioned (though I have no idea whether it has any validity) is that if you have the diacritic on the birth certificate then it's easy to drop it later, or not use it in practice, but the other direction would be harder to justify.