His body and a firearm were found in a wooded area following the spree which began in Whitehaven, a coastal town in the district around 560km northwest of London.
"It is being said that he was in a dispute over his mother's will," said Al Jazeera's Tim Friend, reporting from Cumbria county where the shootings took place.
"One of the first victims is reported to have been his twin brother. He then went on to shoot the family lawyer," Friend said.
"Derrick Bird, according to all reports was a very placid man, he enjoyed going to the pub and had recently become a grandfather," Friend said.
"Something clearly triggered a violent reaction in him."
Bird had recently argued with other taxi drivers, accusing them of taking work away from him, Al jazeera's Tim Friend reported.
'Shock to the system'
The area's local MP said the shootings were a "shock to the system" of a close-knit community already mourning the deaths of two teenagers killed in a bus crash last week.
"This kind of thing doesn't happen in our part of the world. We have got one of the lowest, if not the lowest, crime rates in the country," Jamie Reed told the BBC.
Alan Hannah, a witness, said the shooter drove up alongside him at traffic lights by Whitehaven police station.
"I saw a man with a large shotgun and his windscreen was smashed. I drove through the red light to get ... out of the way. I got home safely but was very shaken," Hanna told the Whitehaven News.
The incident recalls previous mass shootings in Britain, which has tight controls on guns.
One of the most notorious was in 1987, when Michael Ryan, a 27-year-old man, shot 14 people dead in the town of Hungerford in Berkshire, southern England.
In 1996, 16 children aged five and six plus their teacher were shot dead in the gym of a primary school in Dunblane, central Scotland, by 43-year-old Thomas Hamilton.
The two incidents led to a significant tightening of laws on gun ownership, with registration now mandatory for shotguns, which must be kept in secure storage.
Handguns were banned in 1997, and semi-automatic and pump-action rifles are also outlawed.