Neat article, for those looking to save a click:

"Urban areas in the UK cover around 16,000 square kilometres. If we distribute the rats evenly across the urban areas, which is clearly unlikely but necessary for the calculation, each rat has a rather spacious 5,000 square metres to roam around in.

Assuming you're standing at a given spot in an urban area you would be at most 164ft (50m) away."