Someone smuggling heroin into the US would expect a sentence of at least 30 years. We happen to think that automatic weapons are at least as dangerous as heroin.
A gun being smuggled into the UK is only going to be used by a criminal (and by criminal, I mean they're committing violent crimes other than ownership of a gun) for killing someone - a gun smuggler, therefore, is providing a killer with a murder weapon with full knowledge of what it's going to be used for.
Smuggling weapons that will surely be used for crimes including murder, and likely enabling many more murders than the smuggler is personally capable of?