Erection date: 1815
No inscription that we could see.
Harrow School was founded in 1572 under a Royal Charter of Elizabeth I so this statue is entirely appropriate.
About this statue Wikipedia says "... originally at Ashridge Park; moved to Harrow School in 1925." And Bob Speel confirms that the statue was made for the then new Ashridge Park in 1815. But Ashridge Park still has a statue of Queen Elizabeth I and one that is near identical to this one.
ChilternsAONB explains that in 1925 the Ashridge house was sold and all its contents dispersed. We can find no confirmation but we have to assume that Harrow acquired this statue at the sale and that a copy was made, at some unknown date, and installed in the original niche at Ashridge. The harrow niche is clearly made for the statue so it seems Harrow knew they would have the statue when the building was designed, possibly as early as 1921. Or there were some significant late changes to the design to accommodate the statue.
Note also that the statue was erected in memory of Claud Hamilton who only died in early 1925. Which adds more questions about what happened when. Perhaps the Hamilton family chose to fund the statue as their memorial to Claud, possibly after it had been erected.
Site: Harrow School - 5 memorials (5 memorials)
HA1, Peterborough Road, Harrow War Memorial Building and Speech Room
In our photo, reading left to right: the WW1 memorial plaque is just to the right of the lamppost; the War Memorial Building plaque is one of those in the flint wall; the redbrick building at the right is the Speech Room - the road-side wall of which has the statue of Elizabeth and the Speech Room plaque is partially obscured by the top of the pig-pen railing end-post.
The Hamilton plaque is fixed inside the building, almost immediately behind the statue.
The Speech Room building was constructed 1872-7, designed by William Burges. The War Memorial building was constructed as a memorial to the 600 Old Boys of Harrow school lost in WW1. More information at the Listing entry.