This is the same in the UK. You'll find that 101mph+ in a 70mph zone is an automatic ban (though magistrates retain the option to give points rather than the ban) and a £500+ fine. Up to 100mph in that zone is generally points (though magistrates retain the option to give a ban rather than points) and a fine of up to £500.
In my brother's case, it's likely that his true speed was something like 145mph, but there was no accurate reading (something about an Ariel Atom is not laser-friendly). The case was heard by magistrates and in the absence of an accurate reading - just an officer's opinion of speeding with no numbers. Unlike some, my brother chose to say that, yes, he was speeding but couldn't remember the exact speed in question due to personal issues meaning he wasn't in a suitable state of mind (he later told me that he had been doing some considerable speed, saw the copper and thought "🤬 him" and sped up - the speed quoted above is merely my best guess on his description and shouldn't be taken as accurate). The magistrates sided with his account of events, but agreed that a speeding offence had occurred and gave him a conviction for SP30 - excess speed in an national speed limit zone - with 6 points on his licence and a £500 fine. The speed was given as exactly 100mph (see above).
So in this case the eyewitness could have made a pretty good guess for his precise speed but legally could only offer his opinion that the speed was excess for the limit.
We also have a guideline in force in the UK from the Association of Chief Police Officers that no-one should be charged for any speed of up to the speed limit +10% +2mph (30 limit = 35mph; 40 limit = 46mph; 70 limit = 79mph. So you can peg your cruise at that speed on your satnav and clip past coppers with impugnity (a fact not known to anyone on the A1 two weekends since, as they saw a panda car and all stamped on the brakes to 69mph. I passed him in my bright red phallus-substitute and dragged an otherwise cowardly Audi TT with me - I'm always the guy that passes the police car at whatever speed I happen to be doing rather than stamp on the brakes). It is however, just a guideline, not a rule.
So to get to court in the first place, you need an opinion from a police officer that you're doing at least 80mph - and most forces will completely ignore you to 85mph. It's actually quite rare to see a case based on an opinion of speeding rather than evidence of it, and it generally happens in cases where the speed is north of 100mph. Which, I think you'll agree, is a speed even the dullest highways-trained copper should be able to judge...
A key difference, I note, is that you only lose your licence once magistrates decide that you do, not from the instance of the offence. The police cannot judge an offence has occurred, only that they suspect one has. It is for a court of your peers (in this case a magistrate's court, rather than a trial-by-jury) to decide that and your punishment, if any.
The exception is speed camera offences. Which are kinda clear-cut.