Now that's one heck of a piece of trivia. However did you find that out? (Enquiring minds want to know!)
I can't speak for AmericanMother, but some of us have had to be trained in all these protocols and ceremonies.
AmericanMother is quite correct about the passage - it is read at every Service of the Most Noble Order of the Garter. It is considered to sum up the noble ideals of Knightly Chivalry that lie at the core of what that Order is meant to stand for.
There is no requirement that it be used at a funeral of a Garter Knight or Lady Companion, but it is a traditional choice, and Lady Thatcher was, rightly, proud of her elevation to the highest of England's Orders of Chivalry.