I wonder if:
A) The speeding offence was not on a calibrated instrument so would be challengeable.
B) The alcohol offence is marginal/on count back, may be challengeable.
Hence the delay whilst prosecution likelihood was considered.
Purely speculation on my part BP, but I suspect the speeding offence is nailed on. It's not difficult to see that someone's doing over 70 mph, & all they need is the witness statements from two policemen.
The alcohol offence has to be supported by instrumentation, & it's possible that, either the correct procedure hasn't been followed or the readings are so borderline that a prosecution would be risky.
On the other hand they may have him banged to rights.
The article from the Manchester Evening News is the least emotional & the most informative, but doesn't say he's been charged for drink driving, that's why I'm asking the question.