Sarah,
Sadly all these posts are correct. Whilst you may not be high risk, anything apart from the absolutely normal takes you out of the low risk category.
In general, a high risk pg is labelled as such because of: some kind of condition in the mother (usually but not always pre-existing, such as high blood pressure or diabetes) that requires more attention during the pg and/or delivery, and conditions that pertain only to that pg--for example, placenta praevia, or a prenatally diagnosed issue with the baby. Because of medico-legal considerations it's now usually an all-or-nothing deal...if you don't slot perfectly into low risk, you must be high!
I had my first VBAC in a freestanding birth centre. There was absolutely no problem doing that (10 years ago) and I was told at the time that whilst the risk was slightly greater than normal, it was still very tiny. When I was booking my second, in another state and some time later, I was told that I would not be allowed to use the birth centre. I made a big fuss and with permission from the consultant high risk OB, was able to book in there. In fact, on the day I could not deliver there because all the rooms were taken! I ended up in the normal delivery area, but in a room with a bath and the midwife with me who had done almost all my antenatal care. My biggest leverage, which unfortunately does not apply to you, was that I had already had one successful VBAC.
I would first of all try your best to book in to the unit you are looking at. If it's not possible, it would be worth asking about other delivery options at that hospital apart from the birth centre. They may have a room or two with a bath, for example. If you find out exactly what the guidelines are (and who is authorised to bend them!) you have made a great start.
Also, perhaps you can wangle care from the birth centre team even if you don't deliver IN the centre?? It depends what their setup is like...at the teaching hospital here, the birth centre not really separate, it is in a corridor of the normal delivery area...so a great deal of flexibility is possible.
Finally, keep in mind that you can bring the birth centre mindset with you even if you don't give birth in your first choice unit!!