Because it’s very liberating to be naked.
Anywhere, as long as the venue/landowner is naturist friendly and that the staff on duty (if there are any) are aware that there will be naked people.
For nude ceremonies in the UK aim for late spring and summer, and always have a contingency plan; somewhere that’s under cover and big enough for the gathering in case it turns cold or rains. Consider Southern Europe and other warm climate countries which have a longer sunshine season, and don’t forget the sun cream!
Does everyone have to be naked?
No. Clothes optional is a better way to word your invitation, then whoever wants to be nude can be. You may request ‘hats only’ or ‘just wear your best shoes’. A ‘black tie’ celebration for naturists adds a taste of sophistication to the event. Make sure all the guests understand it is a naturist event and wont be offended.
It is entirely your choice if the Celebrant is clothed or naked.