Should I use lights in foggy conditions? Today it was foggy, visibility was only slightly reduced, I already wear a high-vis on my bike but I am wondering if it is still worth me using my lights or is that overkill?

I definitely recommend this. Running lights will make you more readily visible as the light will penetrate the fog to an extent. Reflectors and high-viz clothing will be somewhat less effective during foggy conditions. Use caution riding in foggy conditions as drivers may not see you until they are very close.