Not an official source of information though...
From an official source of information.... http://www.ukvisas.gov.uk/en/ecg/settlement/fiancees
"SET1.18 What evidence is required of marriage arrangements?
The ECO needs to be satisfied that it is intended that a marriage in the UK will take place.
The law relating to marriage in England and Wales does not allow for any arrangements to be made with a Registrar until the foreign national has arrived in the UK. Of itself, a booking at a Register Office or church is not proof that a marriage will take place.
The ECO can reasonably expect the couple to have made some tentative plans for the wedding. Any evidence that may be available that wedding arrangements are in hand may help in this respect."
It does not state that you have to provide provisional bookings. The fact that many of us (including Bluebirdjones and myself) got fiancee visas easily without provisional bookings proves this. Our tentative 'plans' when we applied were that we going to get married in the UK in May.