It is likely that Heathrow will use all the space granted, then build a shorter runway not crossing the M25 after the costs overrun, and use it for short haul flights only. Then build on in the future when viable.
It's possible but I would think this could happen only if it was constructed in such a way as to handover a shorter runway early in the project and the M25 section continued to be built. I will be interested to see how this comes out of the tender process where the shortlisted companies will compete to improve the design and program of works.