Someone with much too much time on their hands

My guess is that it's some hardcore petrolhead who's opposed to speed limits who believes that altering the sign means that it no longer has any legal status.

I'd call them a hardcore idiot

Has there been an almost universal reduction from the NSL (60) to 50 on single carriageway A roads as in Oxfordshire?

A couple of years ago 40 and 50 limits spread like a virus across many roads in NE Wales, until the A5104 is (I am pretty sure) completely limited from Penyffordd to Corwen, where just a year or two earlier it was all NSL but for a small village or two. They were closely followed by average-speed spy cameras. There is no obvious need to control the speed of cars, so I presume it is to make bikers pay for their fun.