The JFK Memorial & Magna Memorial, Magna Carta, Surrey, England.

The British memorial to the assassinated President Kennedy was jointly dedicated in May 1965, by Queen Elizabeth II and Jacqueline Kennedy, prior to a reception for the Kennedy family at Windsor Castle. The memorial consists of a garden and Portland stone memorial tablet inscribed with the famous quote from his Inaugural Address:
"Let every Nation know, whether it wishes us well or ill, that we shall pay any price, bear any burden, meet any hardship, support any friend or oppose any foe, in order to assure the survival and success of liberty."
Close by is a memorial (to the Constitution) tree and then the beautiful Magna Carta Memorial.
This memorial is of a domed classical style and contains containing a pillar inscribed "To commemorate Magna Carta, symbol of Freedom Under Law". The memorial was unveiled on 18 July 1957 created by the American Bar Association (ABA) to a design by Sir Edward Maufe R.A.
Location: 51.44452 -0.56600

[@t.mitchell9135]

Pretty sure that while the US may own the land, Britain retains retains sovereignty over it. Thus, any crime would be handled by whatever British police agency has jurisdiction.