The UK hasn't decriminalized pot - but it's being put into a lower classification, along with steroids, rather than with heroin. That means reduced penalties. And in practice the police in some parts of the country are turning a blind eye to its use in many circumstances.
Having small quantities for personal use won't mean you get arrested, they'll just confiscate it. (And smoke it off duty?)

The basis rationale for this is to stop wasting police time, and the pressure for the change has come largely from the police.

It's a typical British fudge - and since it'll still be illegal to sell it, with increased penalties, people are still going to have to rely on illegal sources, which will often mean dealers who will cheerfully supply hard drugs as well.

And growing your own is still seen as seriously criminal. Which is of course very helpful to dope dealers, because it keeps them in business.

..........

"...if anyone out there reading this thread has a child with a hard-drug habit, what drug would you say from experience was their initial entry-point into habitual drug abuse?"

On the basis of years as a social worker I'd say in almost all cases that would be glue, tobacco or alcohol.