So if I were to put nothing down and plead not guilty to both I am assuming this will be acted upon in court where the police will put forward their evidence (speed + photographic evidence).
No, they won't bother with any of that as they know they have no evidence as to who was driving, proving the car is speeding would then be a total waste of their time, so they won't. This was already pointed out by me.
How would a plea work if I were to accept guilty plea for the offence but not the FTP? Is it favourable to put this forward now vs waiting until court hearing?
You make it clear that the plea is conditional on them dropping the FtF, you can do this in writing by pleading not guilty to both and writing in the 'reasons' box for speeding that you will plead guilty if the FtF charge is discontinued. This was also laid out by New Judge above.