You can't see how Russian intelligence might be clearly motivated to add an ex-agency traitor to a growing list enemies of the state killed when they think they're in safe overseas exile to send out a nobody can escape us message even though it didn't think it worth lobbying legislators to rewrite sentencing laws to do things by the book back in 2006? Really?

The UK government can be wrong, but it the absence of any plausible alternative candidates, it's probably right to conclude the perpetrator is the party known to have carried out similar attacks against similar targets using similar deniable but too sophisticated to be regular criminal means using a murder weapon identified as one they are known to have created. Especially when their public response is trolling and branding the victim a traitor rather than alarm.