It's a Catch 22 situation.
Yes, you can "book" a room for the ceremony at the registry office, but
that's like booking a restaurant or cinema ticket..... there's no guarantee that you'll use it.
So, is this "proof" ? ...... you tell me !
The "Notice of Marriage" is the important thing, but
All civil and non-conformist marriage ceremonies must be preceded by civil preliminaries. This involves giving a Notice of Marriage to the Superintendent Registrar of the district where you live. Notice must be given by the prospective bride and groom in person. The Superintendent Registrar will ask you questions about your identity, age and freedom to marry. You will be asked to produce documents such as a birth certificate, passport or deed poll so that we can check your ages and the spelling of your names. If either of you have been married before you will need to produce a death certificate or divorce decree absolute to confirm your freedom to marry. If your divorce was granted outside the UK it may be necessary for us to submit it to the General Register Office for final verification. Other documents may be requested depending upon individual circumstances. Please bring original documents with you to the Register Office, with certified translations if applicable. Photocopies are not accepted.
In order to give notice of marriage both of you must have lived in a registration district in England or Wales for seven days immediately prior to the day on which you give notice and you must have a marriage ceremony booked. After giving notice, your application is displayed on the Register Office’s public notice board for 16 clear days, so you should allow for this length of time when planning your ceremony. For example if notice is given on 1 August the marriage can take place on or after 17 August. After this period two blue certificates, which allow you to be married, can be issued.
So, effectively, your future wife needs to be in the UK, with a fiancee visa in her passport in order to proceed with the "Notice of Marriage"
Regards
