Do yourself a favour and take the course.
If you go to court you will have to plead not guilty. The issue will be that you believe the measuring device was not accurate enough to record your speed with any degree of certainty. Your basis for this will (presumably) be because its calibration certificate was about to expire. The onus then shifts to you to produce evidence that the machine was indeed inaccurate (the prosecution discharged their responsibility by having supplied you with the details of its calibration). Are you confident that you can do this?
Your contention that 36mph is the lowest speed that action will usually be taken is correct. The Association of Chief Police Officers’ guidelines say that a margin of Limit +10% + 2mph is allowable. However, this is allowed precisely to avoid frivolous challenges (such as you are proposing) to prosecutions on the basis of inaccurate measurements. The limit is still 30mph which you have exceeded by 6mph (20%). You will have to show that it is more likely than not (“on the balance of probabilities”) that the machine was not sufficiently accurate to prove that you had exceeded 30mph, not 35mph.
If you are convicted (which is almost a certainty) you will face a fine of half a week’s net income, prosecution costs of around £350, a victim surcharge of 10% of the fine and (if the offence was committed on or after 13th April 2015) a “Criminal Court Charge of £520. Oh, and of course three penalty points.