So you're in a shed of car with crap brakes and thought it would be a good idea to do 80mph?!
They don't have to allow anything. Speeding is a strict liability offence.
Plus what was your speedo reading?
Speedos are known to over read by 5-10% at 70. My Zed at 80 on the dash pod gives a GPS verified 75mph. Some cars are far worse.
If you were doing 80 it's safe to say the speedo was reading 85mph minimum.
Now I'm not innocent so I'll say no more.