I work in a registry office so I can tell you how it works here (Scotland). You can make a provisional booking well ahead of time. We've had couples book two years ahead! We just have them send us a letter with their names, the date and location of the wedding, and a deposit. We keep a notebook with the details and the information is held until the wedding unless the couple cancel. If the date of the wedding is getting close and we haven't received the notices, we'll get in contact with them.
You have to give notice 14 clear days before the wedding, but we like to have all the documents well before then in case we need more information or clarification. This is particularly true for foreign nationals. In fact I'm tearing my hair out now trying to get a Ukrainian bride to send in her documents - the wedding is coming up very soon, we'll need an interpreter because she doesn't speak English, and she basically has a week to get it all to me.
The best thing to do if you have questions is just to phone up the registry office that will be handling your paperwork, or even better make an appointment to see the registrar. They should be able to answer your questions.