Opportunities
Probably because there is a lot of fat on a £18 billion contract with the inevitable overruns and renegotiations that will expand the budget and keep thousands of civil servants employed for decades along with offering consultancy roles for ex-ministers and civil servants along with their families and friends.
Whereas £1.2 billion offers far less opportunity to hide cronyism and therefore is considered far less attractive by senior civil servants and ministers.
These factors are far more important than any advice from technical experts brought in to evaluate such schemes.
Biting the hand that feeds IT
