Just to add my thoughts for what they're worth. Have to agree with the other posts: Kheo pads don't seem to offer a huge amount of protection. Have used the Kheo pads in the past, but now have a set of pads from halfords, which I find fit more securely (they don't just use velcro straps, also have neoprene backs), additionally they have lots of padding so are able to deal with that jump when you crash onto your knees.
I think a set of knee pads is about £30, pretty expensive but in my opinion well worth it. I think that they are marketed for BMX riders. They are made by sixtone or something, can't remember exactly.