Another thing to look into apart from the speed of the wifi bridge that MacNCheese is suggesting is the cost of streaming all that data over your Internet connection. You probably don't have unlimited data included on your monthly bill so everything over a certain point will cost more money. Find out what your data cap is, how much extra they'll charge you per megabyte, and do the math for how many megabytes you will be streaming per month at .390 megabits per second. 8 hours per day 5 days a week? 24 hours a day 7 days a week? You'll also probably be billed twice- once to send it from the small office and once to receive it at the big office. You may find that the money it'll cost you to set up a wireless bridge will be paid for in the first month or so or Internet overage charges. The bridge should be a LOT faster, not tie up your current Internet connection, and most likely be a lot cheaper in the long run as well. You might even be able to eliminate an entire Internet bill by running the small office Internet along with the camera traffic through the bridge to the head office. Just plug the small office end of the bridge into a wireless router. Distribute Internet and connect your ip camera from there. This all depends on whether or not you can see the small office (or an antenna tower on the roof of the small office's building) from the head office.
If you can't do wireless bridges for whatever reason, maybe have the camera record in high quality to a local computer, NVR, NAS or internal SD card on-site and try to view the substream (lower resolution and quality) remotely at the head office, possibly even on-demand so it doesn't use data constantly. .390 megabits/s is almost 50kB/s and that should support a low-resolution substream fairly well, especially if you'd be happy at dropping your frame rates down to 7 or 4 fps.
