Box Hill I imagine would be fine but it's Leith Hill I'm not sure about. I live in Bromsgrove so probably won't be able to make it down to check it out
Leith Hill is short, you can always walk. Depending on where you are in the crowd you might have to anyway, it gets all choked up with walkers and zigzaggers unless you're pretty early on.
Would not fancy Leith Hill on that, but then I'm crap at hills. Go and try Leith and Box Hill if you're not sure.