I guessed that these stones were once part of the original entrance to the barracks, but it’s disappointing that there isn’t more information about them actually at the site.
Fortunately, they were listed in 1982 and so there’s a description of them on the Historic England page. The text reads:
“Two carved stone plaques, mounted on low bases, being the surviving elements of the former gateway, at this point, to the East Cavalry (Warburg) Barracks. Rectangular white stone blocks, with cambered top edge. In high relief carving, each has the initials VR (Victoria Regina) enclosed within an oval wreath of oak leaves, with crossed ribbons; on each side a branch of laurel leaves. At the foot, incised letters with the words, in one case ‘EAST CAVALRY BARRACKS’, and in the other ‘ERECTED AD 1859’.”
And here’s what they used to look like when the barracks were still standing.