It seems there are different methods, depending on location in the country.

Page 5 of this document gives a few different examples of how it would work.

Edit: its not letting me attach the pdf from my phone. If you google 'bus station costs working paper' - you should be able to find the document by the Competition Commission
This is it: http://www.competition-commission.org.uk/
assets/competitioncommission/docs/pdf/inquiry/
ref2010/localbus/pdf/
bus_station_costs_working_paper.pdf