The idea predates England.
So far as we can tell from records, English kings used their arms not their feet.
The length of one foot has been redefined many times since the last recorded use of an English monarch's body.
In antiquity it was common to use human body parts to measure lengths.
The foot as a unit of measure predates the establishment of England as a country - and hence predates English kings.
Wikipedia cites Oswald Ashton Wentworth Dilke (May 22, 1987). Mathematics and measurement as saying that the Romans and Greeks used the foot as a unit of measure.
So a 12-inch foot predates England. However the exact length of a foot has varied over the centuries. At times, there are records that indicate that the length of a foot was redefined based on the physical size of an actual foot of a ruler such as an emperor. For example:
English Kings did redefine units of measure from time to time. Sometimes they are reported as having used their own bodies as the basis.
The British institution responsible for measures is that National Physics Laboratory (NPL) which says
A traditional tale tells the story of Henry I (1100-1135) who decreed that the yard should be "the distance from the tip of the King's nose to the end of his outstretched thumb".
The 12th century William of Malmesbury's Chronicle of the Kings of England reported that King Henry I redefeined the ell in terms of his own arm.
So the basic idea is sound. But the foot in use today has much more recent definition (example) and likely differs from the actual length of any appendage of any particular English monarch.
The most you can say about the current length of a foot in relation to an English monarch's body is that it probably hasn't been redefined very many times since Henry I implicitly defined it as a third of the length from his nose to his outstretched thumb.
His actual feet were probably not involved in the definition.