Hi there rasp_berry, welcome here to the forum.
May I suggest you take a look at this thread
"How To Apply For A Fiance(e) Visa"
It may provide you with some useful information.
Regarding evidence or proof of intention to marry, the UKBA just wants to see something that demonstrates that you've both discussed some plans about the wedding and future arrangements. It could be facebook messages, e-mails or texts that specifically refer to wedding plans. Such discussions might include the where, the when who will be invited, what to wear etc. Some folks have even included quotations for cakes, reception venues and flowers etc. Of course in themselve, none of these are solid proof that you will actually get married, but it does show 'intent'.
A significant contributing factor is the evidence provided that shows the relationship is genuine. If this is weak then the ECO will start to doubt other things as well, such as intention to marry.
I'm sure you will be OK, many have been down this route without issues
Good luck
It really isn't possible to book/confirm a wedding until you are actually in UK.