I'm thinking about doing the V3K, and I'm assuming Welsh summer weather in the hills will be pretty similar to Scotland. I find summer hill running usually keeps me pretty warm, so I don't have to wear too much.
For upper body, often just a vest, or maybe a T-shirt. Or if a bit colder, a long sleeve base layer, with a vest over the top. For a longer run like the V3K, it might be worth taking an extra layer, in case it gets colder - maybe a long sleeve shirt, or a lightweight windproof jacket/gillet.
For legs, I find shorts (baggy) are fine for most summer weather, I have some longer shorts for colder days. If its really cold, 3/4 length tights or tracksters.
Plus a waterproof jacket and trousers - I carry these in my bag, but hardly ever wear them. And some sort of hat / buff and gloves can make a big difference.