The usual advice here is to have a booking for the wedding, reception, caterers, etc and to be able to produce letters and provisional bookings for these items
The ECO has to believe that there is an intention for you to get married within the 6 months period of the visa.
I have to say that we got our visa without this. We mentioned our intention to get married in the covering letters and the likely date, size and venue. It was an area where I think the application may have been weak.
Had Daisy been called for an interview she would have been able to produce a letter with a provisional booking from the registry office - I only arranged this after the visa application had been submitted.
As it turned out we had to change the provisional date anyway.
