Here's a recent English case, where a mother wanted to call her two children Preacher and Cyanide.

(This is the appeal) http://www.bailii.org/cgi-bin/format.cgi?doc=/ew/cases/EWCA/...

Here, the mother was already involved in care proceedings, and the local authority child protection social workers weren't able to just force her to pick different names, but had to go to court in front of an independent judge.

It's a pretty detailed document and it shows, I think, how uncomfortable English judges are with exercising this power.