I think if you have a look at the service manual[0] you'll be surprised. New code is open by default (unless there's a security risk) and while the stack beneath the surface isn't predefined the frontend is[1]. There's also the GOV.UK PaaS[2] that is geared towards making a fully automatic pipeline an easy process - it allows anything from static sites to Docker containers to be deployed with just a few lines of config.

All services need to go through an assessment where their openness, usability, accessibility and security are reviewed.

It's a modern, sensible approach that's constantly being improved.

[0] https://www.gov.uk/service-manual/service-standard

[1] https://design-system.service.gov.uk/

[2] https://www.cloud.service.gov.uk/