There's nothing in health and safety law to prevent anyone from celebrating the Royal Wedding - in fact, HSE encourages everyone who wants to throw a party to go right ahead.
If someone tells you that you can't have a get together to mark the nuptials of Prince William and Kate, then challenge them. Health and safety is about looking out for any legitimate things that might spoil people's fun on the day, not to stop people doing anything at all.
If you want to close a road or are running a party on a commercial basis with alcohol on sale, it's probably a good idea to check with your local council. But in most cases, things like street parties run by volunteers don't have any obligations under health and safety law at all.
Here's to William and Kate!