I’m planning a solar panel / 12v Battery solution for the lighting in my soon to be delivered log cabin. As the ceiling is fairly low, I’ll be putting a wall light in and I’m assuming that I can get any wall light in as long as a) it has no electronics / transformers in it and b) I use a 12v appropriately sized bulb. Is my assumption correct?

Pretty much. You’ll have a voltage controller which the solar panel/battery plugs into and the light will feed from this. The normal procedure is to size the panel/battery to the demand. So, if you’re having lots of lights you’ll need a battery that can deliver that power and a set of panels that can replenish the battery in time.
If you’re going off grid in a bigger way you would be looking at an inverter to give 240v.
Rich.

Unless you are covering the roof with pv you’ll be wanting LEDs…

12v LED for the win. Just find the right sort of light for what you are doing out there.
You do want to work backwards from that. Knowing the load is the first thing to do when designing off grid power. It helps get the battery and charging sized properly.