When the major 3rd party is the supplier of your core business application and also part of the same group of companies following the purchase of Sabadell, I would be cautious about this being caused by not bringing things in-house.
Ambitious time frames, poor capacity predictions not accounting for new platform requirements or changing business requirements, poor understanding of the magnitude of the change inherent in a new architecture/platform, software bugs in an untested platform (I'm including this through lack of in-depth information - I would expect this in any implementation...), deliberately scaling back testing to achieve a pass and poor governance both at project level (they weren't able to hit their project deadlines but scaled back/incomplete testing results to achieve a pass, going live without adequate capacity for customer services) and corporate level (the boards desire to avoid additional payments to Lloyds seemed to be the overwhelming driver for the project deadlines).
In summary, they attempted a 5 year project (my estimate based on it taking an additional year to get close to fully operational and didn't carry out suitable testing) in 3 years, delivered it in 3.5 years and cost the bank customers/growth for at least an additional year, cost an additional £200m plus any future fines/regulatory costs and reputational damage that will last years.
