For newbies reading this:
@Oldvatr's post is a good argument in favor of self-testing. Indeed, you could probably binge on carbs for a week (especially right at the beginning of a three-month gap between HbA1c test) and it would have a minimal impact on your test, indeed it could have no impact at all if you time it right.
I think it is likely that a true "binge" over a period of one or several weeks would be harmful to your health, even if the regular A1c is stellar. This is especially important if your doctor has decided to space out the A1c to six months, or a year, for instance. In these circumstances it is up to us, either to show better discipline and/or to self-test.
As someone who does not self-test, I have been making sure that the clinic A1c is run at least every three months. In the long term that becomes harder (here in America) because the insurance company will eventually demur. From my reading on this forum, it looks like a similar situation pertains in the NHS. They want to save money and time, so they lose interest in "well controlled" T2 diabetics. Edited to add: Which is silly given how simple it is to take a blood sample, and how relatively cheap the A1c test is, nowadays.