In this case they already know who the 1 in 5 are, RedToothBrush:
Kadcyla is used to treat people with HER2-positive breast cancer that has spread to other parts of the body and cannot be surgically removed.
About a fifth of breast cancer cases are HER2-positive, and it is thought this drug could benefit 1,500 women a year. (from here)
However, £90,000 for 6 months comes out as £180,000 / QALY.
Generally if a treatment costs more than £20,000 to £30,000 per QALY, it would not be recommended as cost-effective by NICE.
Although for end-of-life treatments NICE will go to twice that amount. (from here)
Even if you take the higher figure of £30k and double it for an end of life treatment, this drug costs three times the usual threshold.