I'm quite negatively surprised that a government service is moving from their own platform to AWS for such an important service.
I don't know what it's like in UK but it may be the case that government has a hard time a{ttract,fford}ing talent to administer everything in house. Not that AWS is great for cost saving but if its between paying 50k/year for cloud services and not being able to find an engineer who will competently do the job for less than 50k, then the cloud is your only move really.
Everyone I know working FedRAMP jobs is prior military/g-level.