As stated above, the speed limits for the UK is 70mph on motorways and dual carriageways unless there are signs to the contrary, so go to the stretch of dual carriageway and see if there are any signs there if there are not then you are not guilty. If you have a dash cam, repeat the journey with it on, or have a passenger video it as you drive through as proof

Here you go https://www.gov.uk/speed-limits