Nowadays, Scottish Country Dancing tends to be more formal.
My mum was born in the 1920s, and in her day there was no discernible difference between the two - other than the fact that SCD might include some more esoteric dances that you wouldn't hear at an ordinary ceilidh.
[At some point, someone will point out that strictly speaking a ceilidh isn't a dance - it's a gathering where people take turns to perform, but the understanding of the word has changed over the years.]
The current trend for ceilidhs is for the pace to be a bit faster than was formerly the case. If you ever listen to Jimmy Shand (Snr, not Jnr) his timing was impeccable and much more sedate than the music you'll hear from modern ceilidh groups.
I'm nearly 66. The generation above me used to complain about the bands coming out of the university circuit in Glasgow, saying that they played too fast - but this was what more modern participants were looking for.
Paradoxically, the Royal Scottish Country Dance Society actually devises new dances in the old style.
In short, I'd say that originally there was scant difference between the two. If you find yourself at a formal event run by the RSCDS, the dancing style tends to be stiffer and more twee with a more precise rhythm IMO. (I'm waiting for someone to give me a slap for saying that.)
A modern ceilidh tends to be livelier with a looser style of dancing and (again IMO) more of a focus on fun rather than getting the steps absolutely right.
sits back and waits for pelters
ETA Youtube does have various instructional videos which might help you.
As for learning in Devon, there might be a local Caledonian Society that could help you out?
ETA Again
If it's Highland Games, I'd say that it's going to be more informal, particularly since many people will be fairly pished after visiting the beer tent relaxed.
[Don't ask about what happened to my late husband at the age of 15 at Fort William Highland Games.]