What happens in the rare situation where a Contractor decides to accelerate (small a) by his own accord? Looks to me like nothing happens other than planned Completion comes forward on the programme for acceptance and if realistic etc the Employer should accept. This is under an option C contract.
Yes that is pretty much it. Under option C both Parties share on any cost savings made due to finishing early, but don’t forget they may have spent money to “speed up” (which hopefully was less than the prelims saved).
The Contractor would do this under any option to de-risk the project and to make it less likely they would incur delay damages. Planned Completion coming forward creates “terminal float” which the Contractor owns.