At iRV2.com, in the Dinghy section, there is a long thread about the battery disconnect on a Ford Focus, which has the same requirements as the Fiesta. A couple of posters came up with very good, solid hardware solutions to make it easy. Because on the Focus, the negative terminal of the battery is quite hard to access. I don't know if the Fiesta is the same.
Some people took the easy way out and put a disconnect at the firewall ground terminal. But they didn't realize that one or more computers were still connected by small wires at the battery. That leads to transmission destruction. So the disconnect has to be at the battery.
Overall, yes, you would simply reconnect the battery and go as needed.
I would add, drive a Fiesta AND a Focus. The Focus is still a light car, and it is a lot more comfy and useful than the Fiesta IMO.