Won but Died

Johnny Owen ?

 

Owen did die, but he lost the fight.

 

Recently Yo-Sam Choi died after a 12 round decision which he won, although I'm not sure if it's the fight you're looking for.

 

Was this a alpha title or lineal title?

 

Was the loser of the bout disqualified for a headbutt?

 

William Wallace, he died and then won though

 

Felix Bwalya beat Paul Buke over 12 and died a few days later. Commonwealth LW title.

God rest his soul

 

But he was DQ'd?

 

Felix Bwalya (born October 27, 1970 in Chingola – died December 23, 1997 in Lusaka) was a boxer from Zambia, who competed for his native country at the 1992 Summer Olympics in Barcelona, Spain. There he was stopped in the first round of the lightweight division (– 60 kg) by Tanzania's Rashid Matoumla.
Nicknamed "The Hammer" he made his professional debut on July 17, 1993 in Lusaka. He won the African Light Welterweight Title in 1995 by defeating Gashaw Gudeta. After having lost in the fight for the Vacant Commonwealth (British Empire) Light Welterweight Title in early 1997, Bwalya fought Paul Burke from England for the same title on December 13, 1997. The Zambian boxer won a unanimous decision in the 12-round bout despite being knocked down and saved by the bell in the last round. Afterwards he fell into a coma after what colleagues said was a drinking spree following his controversial victory over the Briton in Lusaka.
Bwalya, who was on medication for malaria, went on a two-day celebratory binge soon after the fight, friends said. He complained of a headache and was taken to the intensive care unit of the local university hospital, where he lapsed into a coma. Ten days after the fight he died from head injuries sustained in the match, aged 27.

 

Mark Goult died the night he won the Southern Area title against Danny Porter; but that was a 10 rounder.

Bradley Stone aside, I know of no other fatality in the era of 12 round British title fights...