My guess is that they would check what your cover was for the previous years and act accordingly.
So if lets say in my case where I clocked 45k miles in two years if I was covered 10k each year then they could say that my cover was invalid all this time?
I don't know; but better safe than sorry.
By the way I trialed what you mentioned with the mileage on the LV site since it is the quickest one to generate a quote. Went from 25k miles per year to 10-11. Price dropped from 1800 to 1600£....LOL.....