I'm no tax expert but I do pay it.
A basic google indicates it costs around £40,000 per year to keep a Prisoner incarcerated.
There are around 30 Million income tax payers in UK
A quick calculation indicates that If we did exterminate him it would probably save UK tax payers about 1/10th of a penny each per year. Let's say your lucky and retire in 10 years. You've paid around 1p to keep a serial killer off the streets for fucks sake.
So stop using tax and money as an excuse to off Brady. Just leave the fucker where he is for Christ sake.