Back to registry office weddings...you don't have to get married where you live, but you do have to give notice there. For example, we live in Birmingham, so we gave notice at the Birmingham registrars, but we got married in nearby Solihull because we liked their facilities better and there was more parking nearby.
There was a lot of leeway in what we could have as our wedding service - it seems like for the vows, etc., there were five different 'formats' in the wording that are considered legal, we could choose that which we liked best. We could also have our own reading if we wanted to - there were a number to choose from, and we did chose one from their offerings, just like we chose music from their offerings. You can choose your own for both, but they vet it to make sure there is no way there is any religious connotation.
Only your invited guests can come in to the ceremony, i.e., no one wandering in off the streets! The room was booked for our use for 1/2 hour - that included the ceremony and some time hanging around afterwards for photos - it was a nice ceremony, and we did most of our photos out front in the area where there were lots of roses growing. And then we partied elsewhere...it seems like we had a choice of three different rooms for the ceremony - we chose the smallest, which would seat 35...