Hello.
I think that you may have made the right choice for you. However, do not base your decisions on your fear of Physics/Electronics or the fact that you have not visited Exeter. If you chose Cardiff because you are hoping to avoid Electonics, then you meet it, you would be pissed.
For the general engineering course at Exeter, I think that the only issue is that you would take modules in subjects that you do not find interesting; but this would for a short time. This is because you would be taking more modules in mech engineering after your first two years. I think that this course would be better especially if you have not done Physics.
I think that it is important for you to visit Exeter before making your decisions. So that you are certain that you made the right choice, rather than thinking "what if?"
On that note, Cardiff is not a bad university, as you said. It is a Russell Group university which shows its academic strength.
I do not think that you would go wrong in any university, but you need to make the right choice.
Good luck.