A few comments on the comments:
"That seems really petty given your clean track record, 1mph over the "10% and 2" "
The speed limit is 30mph and you can be prosecuted for any speed in excess of that . The "10%+2mph" allowance which Answerprancer mentioned is provided to avoid frivolous challenges to speeding charges by suggesting the equipment and/or method of measurement is inaccurate.
Which leads me on to:
“Ask them when the machine was last calibrated. “
If you are offered a fixed penalty (or a speed awareness course) you may get an answer to this question but, as you have seen, there is no provision within the paperwork for acceptance of a fixed penalty to do so. You may lose the right to a fixed penalty and instead have to make a court appearance where the penalties available to Magistrates are considerably higher. You will be wasting your time and jeopardising your chance to a reduced penalty.
As far as your insurance goes you should inform them whether you are offered a fixed penalty or a course. Most insurers ask, on their proposals, whether you have any convictions or have been offered a speed awareness course and expect them to tell you after cover commences if you attract either convictions or the offer of a course. One instance of speeding does not usually lead to an increased premium with most insurers, but if you fail to inform them and have an accident you may find yourselves without cover.