No, K2 is much more harmful than LSD and marijuana. And it never would have caught on if marijuana were legal.
Other drugs in class B are: Amphetamines, barbiturates, cannabis, codeine, ketamine, methylphenidate (Ritalin), synthetic cannabinoids, synthetic cathinones (eg mephedrone, methoxetamine)
Theoretical sentence for possession is Up to 5 years in prison, an unlimited fine or both
And theoretical sentence for distribution is Up to 14 years in prison, an unlimited fine or both
This specialist solicitor has some case reports of mostly large quantity: http://www.mjreedsolicitors.co.uk/news/cannabis-cases/cannab...
So we're nothing like the US (where simple possession could get you a life sentence) but it's wrong to say that possession of small amounts of cannabis is ignored in England.
EDIT: spice is popular because it wasn't being tested for in prisons, while cannabis was; and because it was legal.
To be fair, the life sentences are due to the idiodic "Three Strikes" laws (I really wonder why those aren't considered double-jeopardy). I don't think you can go to prison for life for simple possession in any state otherwise.
Weed is a class B drug, some people get tested for it some don't know where to get it. There is probably a very small market for synthetics in the UK, were weed legal, but I doubt it would amount to much.
I say this as someone mildly involved in the UK RC scene who knows a few users. Most of them would switch to the real thing in a heartbeat were it legalised.
Is the weed situation much more lax in the UK than in the US?
If you get caught, more often than not it is confiscated / thrown away.
Same for growing for personal (no scales, no baggies, no tick list at the property, no "sorting a few mates out" - actually personal), a few plants and nothing will happen, at worst a few hours of community service
And a criminal record, and never being able to travel to the USA or various other countries again for the rest of your life.
Nothing big...
You have a funny idea of decriminalisation.