They must claim different averages on different pages, https://oxfordcycling.uk/about-us/ says 12-13mph.

Yes, of course downhill is faster. On a mountain bike I've only knowingly once managed to break 30mph, but that was going for it and definitely not erring on being safe. My average road speed at the time was about 15mph.

The top speed for powered cycles should be around the normal average as then there's not a large differential between powered and unpowered bikes making it safer when they share road space.