I'm in the US so I can't ...

I'm in the US so I can't speak specifically about how the UK handles things, but from what I know the UK has a number of advanced (state of the art) solids treatment facilities, that pretreat sludge using thermal hydrolysis prior to anaerobic digestion. Anaerobic digestion is carbon negative, but building an advanced anaerobic digestion facility costs millions of dollars, and typically isn't financially feasible unless a facility is processing over 10 million gallons of wastewater per day. This could explain at least some of the trucking of sludge from smaller sites by tankers to larger sites. 

Hope this helps.