70 was in force at the time

what was the speed limit in force at the time that you were on the road?

Your speedo is probably reading slightly high (some quick Googling suggests maybe ~2-3mph). APCO/NPCC speed enforcement guidelines suggest +10%+2mph but not all police forces go by those. If the speed limit in force was 70mph it is going to be a squeaker ...

I would say that most cars tend to read higher than 2-3mph over at 70-80 on the speedo - my car is more like 4-5mph over then, but far less at 30 to 40mph. Satnavs can (if good quality) give a more accurate reading, but even then I wouldn't rely on them to bump right up against either the posted speed limit or the Xmph +5 or 10% +2mph rule for getting nicked or your car's photo taken by a static camera.

In theory, HGV's speedos should be better calibrated, but even then relying on other vehicle's speedos to drive as 'fast' as is legal is probably not the safest or most wise course of action.

I got done doing 31 through a village years ago but thankfully because my license was clean I only got a speed awareness course, I have heard of others who have also been done doing 1 mph over the speed limit.

I have heard people say they have been "done" for 1mph over the limit. I have never seen any evidence of that. All forces in England & Wales operate to the National Police Chiefs' Council's guidelines (of action beginning at (Limit + 10% + 2mph)). One or two extend that to +3mph. Can you remember what force it was that took action for 31mph?

If your speedo was showing 80 then expect a Pixed Penalty Notice through the post.

As I said earlier, Fixed Penalty Notices are not issued for speeding offences. What you (or the Registered Keeper if it is not you) can expect is a Notice of Intended Prosecution (a NIP) accompanied by a s172 request for the driver's details. When the driver has been identified he can expect either the offer of a course, or a Conditional Offer of a Fixed Penalty or notification of court action (mainly depending on the speed alleged and his eligibility for any court-of-court disposals).