Adding an AC circuit to 1997 Scamp 13

[nigeleccleston]

I want to have AC in my Scamp and I'm thinking it would be better to have a separate circuit for the 5,000 BTU window unit I'll probably use. I unscrewed the breaker box (see attached pic) and there is an empty slot on the right.
Can I connect the live terminal (thick black wire) of the existing breaker to the new breaker?
An alternative plan - the existing breaker has two switches. Seems to me that the left side one controls only the fluorescent light above the sink (wires at top). The right side (wires exiting at bottom left) go to a two-socket electric outlet and then come back over to the refrigerator (the wide white wire you see to the left).
If the only thing on the left is the above-sink fluorescent, could I just wire the AC into that?
I apologize for my lack of basic electricity knowledge!

[Gompka]

Nigel if you put in a new breaker, it will automatically get its power from the terminal at the top where the thick black wire is attached and where the existing breaker is snapped in.
You can get either a single or a double breaker, with a double it will allow you for future expansion. It should be fairly simple, just snap in the new breaker, a/c black wire will go to the bottom of the breaker, while the white wire will go to the strip with the other whites, and the ground with the ground strip with the green and bare wires.