The parents would be legally liable in the UK as well, which actually has stricter laws than the US in this regard.
OP, you (and many posters) have misunderstood the case. The legal grounds are being misreported. It's not just an arbitrary moral line like "not raising your kids well".
In the UK, in cases where 3rd parties have equipped or aided murderers, they have sometimes even been charged for the actual crime itself (eg murder, not manslaughter).
- Father bought son a gun as a gift shortly before shooting
- Parents ignored son's requests for mental health help
- On morning of shooting, parents cut short a school meeting about a disturbing drawing their son had made to go to work and declined to take the then 15-year-old home
- Despite mother texting someone that morning that she was afraid her son would do "something dumb"
- Mother said she "didn't feel comfortable" being responsible for securing the gun and left her husband to manage it
They could be liable on many grounds in the UK:
- Criminal: Accessory to murder
- Criminal: Charged for manslaughter
- Criminal: Charged for murder directly, especially if it was a crime pursuant to another crime (if father buying son gun was illegal in that state)
(The law on the above 3 is mixed up and has been a long-running debate – see Chan Wing Siu, Jogee, etc – that hinges a lot on interpretations of foreseeability, virtual certainty, etc... But either way the parents are in jail)
- Tort: Negligence of fiduciary duty of care
The reason the mother and not the father has been convicted is because the PARENTS themselves sought separate trials.
The mother tried to redirect all the blame to the father during her trial... The father is yet to be tried and will probably receive an even harsher sentence for being the one to buy his son a gun as a present.