Destroyers were originally developed as a defence against torpedo boats, and the first torpedo boat destroyer (TBD) in the Royal Navy was HMS Havock of 1893.
There is no official Admiralty order pertaining to the change and the abbreviated term "TBD" is present in the Navy List up to 1919, even though destroyer was the term used in most official orders from 1917.
These are ships that the observer would recognize as being the forerunners of the modern destroyer, with the turtleback and low conning tower replaced by a forecastle and wheelhouse with a compass platform above.
Flotilla leaders were generally named after famed historical (and generally, naval) characters and vessels building for other countries that had been commandeered for the Royal Navy were not allocated into the letter system (e.g. ex-Turkish ships received "T" names and ex-Greek ships "M" names with a Greek mythology theme).
The Type 45 destroyers, the Royal Navy's latest vessels of this type, have resurrected the letter naming convention, taking D names, and repeating some of the previous Daring class.