Surface access includes an awful lot of stuff. HAL get benefits from Crossrail, Western Rail Access to Heathrow, Southern Rail Access to Heathrow, etc. etc. HAL dont want to pay for those, but they undoubtedly help the surface access stats, and thats the kind of support they are looking for.
All the works associated with building the runway itself, including moving the M25, are fully funded by HAL. Im in a better position than most to state that. Also see
https://www.theyworkforyou.com/wrans...0-01-10.1903.h
That said, given the recent Court of Appeal decision, that might move the goalposts significantly in the future.