British ships all have the HMS prefix before their name.
Ammerican USS, Australian HMAS, WWI Germans SMS.

I have sometimes seen WWII German ships given KM as a prefix, but more often than not they don't.

Should they have the prefix?

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"Bringer of Warmth, Carrier of Carrion, Prophet of Dilgarness, Speaker of all thing Llort!"

Part-time Narn.

ACTA playtester
Victorious Grand Admiral

Dont know the answer on that, but I would say for you its personal choice and it makes sense.

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"Official Naval Boffin"

I use DKM, but in reality, not sure they used a ship national prefix in WWII.

They didn't. Germany, France, Japan and the Soviets simply used the ship's name with no prefix during WW2. Some authors have used an historically inaccurate prefix such as KM, KS or DKM for convenience, but none were actually used at the time.

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"Truth is a 3-edged sword" bit like a toblerone, really.