Special shape commercially sponsored hot air balloons have to take the prize for originality. For example:
Bertie Bassett - G-BXAL (Bassett's Licorice Allsorts)
Alka Seltzer - G-CURE
Financial Times - G-ETFT
Ordnance Survey - G-MAPS
Guiness can - G-PURE
Sonic the Hedgehog - G-SEGA
National Power cooling tower - G-WATT
Ice cream cone - G-ICES
Uncle Sam - G-USAM
BBC1 World - G-IBBC
Peanut - G-PNUT
Golf ball - G-PUTT
Light bulb - G-LAMP
Toliet rolls - G-LOOS
J&B Whiskey bottle - G-JANB
Flying Saucer - G-GUFO
Santa Claus - G-HOHO
Humpty Dumpty - G-EGGG
There was a special shape condom around at one point - I'd like to think it had an appropriate reg... G-ROPE or G-HARD perhaps.