Why not just report the flatmate dead but keep the money? The police would likely assume whoever murdered him also nicked the money. No need to hack up a dead body at all
He wasn't murdered though, it was either a suicide or od.
And secondly, the three were a doctor, accountant and journalist. The journalist might not have been raking it in, but the other two can’t have been so poor that however much money you can fit in a suitcase (probably <£100k, split three ways) would be worth risking your whole career for
I think it was more in the region of a million or so but I don't think it's a stretch they'd have committed the crime regardless of careers; they were still all renting so weren't as rich as you think. That and many high paid jobs such as accountants, celebrities etc have being done with tax evasion, robbing clients, fraud etc.
There was a time then where every top British celeb was being pulled before the courts for tax evasion remember from the Beckhams to Gary Barlow.
When it comes to greed the lengths people go to never surprises me.