In 2001, UNIT was involved in investigating an assassination attempt on the Queen in Sheffield. However, despite eyewitness accounts and photographic evidence of bullets hitting her, the Queen escaped without a scratch. Shortly after this assassination attempt, unit forces under Brigadier Bambera are reported to have stormed Buckingham Palace. An account given to us by a UNIT trooper involved in the assault claims that UNIT troops were after two people involved in the assassination attempt, and that they were attacked by a giant fireball. However, the trooper believed that the fireball was some kind of illusion, as it did no damage to anyone. The account doesn't give any idea how the assassins could have ended up in Buckingham Palace.
There are, of course, details that don't add up. One of these is the apparent illusory nature of much that happened. How could an assassination attempt appear to kill the Queen, but actually do no damage whatsoever, and how could a giant fireball do no damage to anyone caught in it? There is also the question of what the alleged assassins were doing in Buckingham Palace (and why they were - as far as we can tell - never even questioned about the incident). If they could get into Buckingham Palace as easily as they seem to have done, then why not attempt to assassinate the Queen when she returned home, or enter the palace when she was home? Basically, the whole thing makes very little sense.