No idea about marriage or a ceremony like a register-office but at a site, you'd be better asking a Registrar, they deal with ceremony as an official legal process which can't be held just anywhere. Handfastings are a form of legal marriage legal in Scotland, check with the PF for details.
However, Handfastings aren't a legal ceremony in England and Wales, so there's a lot of flexibility. I've seen them done at many ancient sites and they can be beautiful celebrations.
Do you follow any particular Path? There's usually someone in each community who is willing to officiate and organise handfastings, or at the very least give guidance, not just on the ceremony but how to respect the site as well.
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