UK marriage legal question… for a fic, of course.

Okay, so I’m working on some various plots and things for upcoming fic, both pro and fan, and I have a question for the Brits and Brit-pickers out there about a story idea.

Can you be married to someone in the UK and not know it? The idea came from a news report that Janeane Garofolo apparently went through what she thought was a fake wedding just for fun in Las Vegas once, and recently learned holy crap, it was actually a legal marriage. (Link to story about it in USA Today: here — and here’s one about whether they have to file amended taxes at the married rate {No, apparently.})

Rather than try to create some excuse to get characters in the Potterverse to Las Vegas, I was wondering if anything like that could happen in the UK? Wikipedia says for a marriage in the UK to be binding, the participants, the celebrant, and witnesses must sign a document… but that’s Wikipedia, and the question I’m actually asking is… what if one of the participants is tricked into signing, i.e. didn’t think it was real? Or it was part of a performance?

Thoughts welcome. This plot bunny isn’t sure where it wants to go exactly…