The first badges of the British Navy were often the Captains Personal Badge made by a Ships Carpenter in King Henry VIII Navy Royal. Through the 18th and 19th Century's British Navy Figurehead's often reflected a ship's name with their official demise in 1894 the Admiralty wanting to keep an ornamental fashion in it's ship's sought badge's.

Badge's were first tabled in 1916 but it was not until 1918 that the Admiralty officially established The Ship's Badge Commitee to design, uniform, regulate approve and issue it's Ship's Badge's.
Leading the way for the Admiralty in 1918 was the Curator of The Tower of London Armouries Charles Ffoukles (1868-1947), Lt RNVR (1914-1918), Mjr Royal Marines (1918-1920), Sgt Home Guard (1940-1945), Hon DLitt, OBE, CB who was appointed by The Ship's Name & Motto Committee (1913) to devise and standardize the Admiralty's Ship Badge's.
In 1935 Charles Ffoulkes retired from his Admiralty Appointment having created 500 badges to become Master of The Tower of London Armouries, his design's were held by The College of Heraldry who became involved with the colouring of badge's and advising the Committee on Coat of Arms and Emblem's.  Sir Arthur Cochrane (1872-1959) suceeded Charles Ffoulkes and in 1983 the MoD merged the Committee with The Ship's Name Committee.

Before 1940 the shape of the badge denoted the type of ship.
Battleship/Battlecruiser circular, Aircraft Carrier/Submarine diamond, Cruiser pentangonal and Destroyer shield.

For security reason's in 1940 the Admiralty ordered that all it's ships will have circular badge's.
A further badge change happened in 1976 which was Royal Fleet Auxillary ship's will have pentagonal badge's and Shore Establishment's will have diamond badges. 2018 will be the Centenary of the British Navy Ship's Badge which is wonderful testament to the British Navy's keeping of tradition, it's Badge's are far the best of any Navy. 

It's not true that badge's of postwar ship's who's predecessor served and survived WW2 are an exception to the 1940 order.
There are 1,655 official British Navy Ship Badge's each one is different, no two are the same. I have not included badge's for Dockyard's, Repair Yard's, Hospital's, Command's. Division's, Flotilla's, Squadron's and Parties in this count as these are not sea going ship's!

Since the 20th Century the British Navy displayed it's Ship Badge's on the ship's superstructure below the bridge screen and it was common to see replica badge's and their image's on the ship's gun's, in wardroom's, mess'es, on fitting's and on it's paper. Over the past decade with the push for new warship's it's become fashionable for the British Navy to display a large 1 dimensional image on each side of a ship's funnel a nice addition which originated in Minesweeper's.

There are 4 type's of replica badge.

Plaster, a cheap, easy gift made by Sailor's and Dockyards.
Herculite, modern eqivalent to plaster, a gift made commercially under MoD license and privately.
Resin, durable made by Dockyard's to ornament ship's.
Metal, durable made by Dockyard's to ornament ship's.

Before today's consumer society replica ship badge's were only occasionally given to VIP's and Organisation's visiting ship's. It's INCORRECT to call these ornament's Crest's, they are BADGE'S!

Is a Deck called a Floor!