Hi,
For our application, which was approved, we just had a provisional wedding reception booked. I couldn't get anything from a registrar, since my fiance would have to be in the UK 8 days before we can book anyhow. The ECO will know this is a law/rule.
In all honesty, I just booked a small venue (£150 deposit) , which we will now upgrade from when Shei is here. At the moment it is difficult to arrange anything with registrars until my fiance is here. The ECO knows this is Law in the UK - so my opinion (in which I might be wrong) is that a provisional booking with a registrar is not so useful based on these facts.
However, some sign that you intend to get married is useful. Receipts for Wedding rings, reception bookings etc are probably more useful than registrar bookings.
