Technically you aren't supposed to book until you have been married, but with the long wait times for appointments, it would be one of the few times I suggest "omitting" that you are not married yet when booking with the IEB. But as the others have said, it does need to be booked for after your wedding. Also, the other PEO's should all be online with the same-day biometrics, so Birmingham, Liverpool, Glasgow and Sheffield all should take appointments and all of them are easier to deal with then Lunar House, so if they are at all closer, I would suggest going to one of them.
Whatever you do do, don't let your wife leave the country while still on the Fiancée visa and try coming back in. They are getting even more and more touchy about that these days and it would be silly to get caught in a technicality for two days in Bruge.
Also, she will not receive a vignette in her passport and will have to wait an additional 5-7 days before her National ID Card arrives. When they scan her passport at the border they will demand she show her National ID Card. So you can't leave the country until she has that in her hands.