It's not silver - if it were it would just have hallmarks. I assume it is silver plate - it would probably be post WW1; McKinley Tarriff Act "Made in England" is generally said to date after 1921.

The trade mark is Dreadnought - both the name and the ship figurative Mark - Googling I have found one other example - Vintage English Chased/Etched Silver Plate Sugar Scuttle/Salt Pig - Dreadnought - but they had no further information. 

990 would I assume be the pattern number - may also refer to the size. If you keep Googling you may find some further information. Good luck.