Even one 'showstopper' is too many to even think of going live when the application is so central to your business.
The real problem with testing large systems like retail banking packages is that they are far more complex to test than the typical bank management ever realise if they haven't been through it previously. Whether the system is running on a mainframe, server farm or in the cloud doesn't really affect the size of this task. Any such system has to handle transactions input via:
* ATMs, web browsers, phone apps, FastPay, SWIFT, CHAPS, Paypal and probably some others I've missed.
It needs to handle various account types:
* Current accounts, savings accounts, mortgages and loans working in multiple currencies
..and must handle complex staff activity using PCs and other terminals with big displays as well as printing fancy statements, etc.
Testing something like this is expensive and time consuming because testing MUST exhaustively cover everything from normal daily operation through account creation, deletion and migration in and out. On top of this there is special processing at month end and year end (both company and tax years). Additionally backups and restores MUST be tested and so must input overloads and auditing.
All this testing needs to be scripted and repeatable simply because its unreasonable, and expensive, to expect testers with phones/laptops/ etc. to do this sort of testing repeatedly without making mistakes.
Sabis already had Proteo, an apparently configurable retail banking package, and so you'd expect a set of development/configuration/testing/regression tools as part of the standard package. However, judging from the way things panned out it either lacked these tools or it wasn't configurable enough to meet UK standards without modification, and maybe some PHB thought tweaking the development etc tools to match UK requirements was unimportant.