However, when applying for a fiance visa, you are required to show evidence that the wedding will take place within 6 months. In my case, I got a provisional booking from the registry. Wouldn't the fact that you had gone ahead and booked a wedding before the application was approved also imply that the application was a formality?
This is somewhat incorrect.
The only time someone needs to show evidence that a marriage will take place within six months is when one of the couple has an impediment to marriage - such as an on-going divorce. In that situation, the ECO can seek assurances that the divorce will be concluded before the fiance visa expires. To satisfy this, the applicant would usually attach a statement from their attorney explaining that the divorce will be concluded soon. A plane ticket will not help in this case.
As to the provisional booking at the Registry Office, the DSP's point out explicitly that a provisional booking is not in itself evidence of planning to marry. Or even a booking with the CofE for that matter. Nonetheless, people do it, including any that I work on. Perhaps it's just out of tradition that people attach these - but in reality, they carry no weight. The statement of intent is all that's needed here.
Despite having said all of this, if there are wedding arrangements, it's good to list them in the statement of intent. But still a plane ticket does not help. Some catering halls need to be booked up to a year in advance. It's more along the lines of a receipts for the arrangements, like invitations, the wedding dress, or a proposal from the caterer - but these are things connected *directly* with the wedding.
I hope that people reading this thread will not interpret that a plane ticket is helpful when applying for a settlement visa. It is not helpful, and the IND advises against it.