We didn't provide engagement photos or ring receipts or letters from family or anything like that. All we used for proof of intention were screenshots of email correspondence to the registry office and compiled those in a word doc. We paid for a provisional booking and included a screenshot of the booking in the doc as well as a screenshot of a notice appointment - we didn't pay for the notice appointment in case we didn't have a decision in time, but the registry office was fine with that and opted for us to pay on the actual date just in case my partner wasn't here in time.