As a Brit living in London, I think it's just not necessary and unlikely to happen. The US has shown that decriminalisation or making it legal encourages use. From my personal experience though, police in London at least don't care much. I've seen a guy smoking a blunt while speaking to a police officer in picadilly, and a guy rolling one on the tube. They go after the distributors but not as hard as with Class A drugs

What's your definition of 'decriminalised' here? e.g. de facto (in practice it is not enforced) or de jure (in law)? Does it being illegal with civil penalties only count as decriminalised?