The operation has changed slightly over the years, but now MKE tends to operate out of a house and a flat, Annabella's out of a number of flats and a house.
For Annabella's my experience has been the better quality / more popular girls operate from the flats in pairs, and the house has a number of girls working from there.
You can book in advance, and then call on the day to confirm when you will be given the address. However it is not unknown for girls to not turn up, even after you have confirmed.
Calling 30 mins after the lines open will occasionally allow you to get a booking with a popular girl when someone hadn't called to confirm.
If you call for a drop in you will usually be directed to the house where the girls are introduced to you one by one and you get to choose - good fun if there are a few free.
With the odd exception I find the better girls are allocated to Annabella's.
Sure there is back to back booking, but I usually find the service list reliable, and for £60 for 30 mins it works for me.