During the hearing, Jonathan Polnay KC, for the Attorney General, argued the 21-year term did not sufficiently reflect “the very high degree of seriousness of the offending”.
He said Digwa had abused the legal exemption allowing Sikhs to carry religious blades, which permitted him to have the dagger, and that this was “on at least one level analogous to an abuse of trust”.
Imran Khan KC, defending, argued the trial judge had counted overlapping aggravating factors more than once.
“To put it pithily, the Attorney General says 21 years was too little. We say it was too much,” he said.
The judges agreed some factors should not have counted against Digwa, including the community impact of the case, which he had not anticipated or intended.
Digwa’s mother, Kiran Kaur, was jailed for three years in July for assisting an offender by removing the knife from the scene. She abandoned an appeal against her sentence shortly before Thursday’s hearing.
Digwa has also applied to appeal against his murder conviction, which will be decided separately.