Fire Plug is an interesting possibility.
My money is still on it being a boundary stone. They come in all shapes and sizes but there are plenty which strongly resemble this one. If it were anything else but a boundary stone why would they have cut both the stone and the new stonework to leave it in position rather than move it a few inches?
The F would be the name of the landowner or possibly the estate or large house which occupied the site or in some cases the name of a Parish or Council.
Here are a few examples of similar boundary stones from opposite ends of the country.
[I can't get the last image to display -so click on the url]
http://tinyurl.com/hezl7u2
It used to be said that the statue of the Black Prince had been placed in City Square , near the station, pointing South to tell all the southerners who've just got off the train to b****r off back down south!