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To expand on the other answers a bit, not everything on .gov.uk is actually ran by GDS, but rather GDS provides governance.

I worked on building out a kubernetes platform for a large Government Department. Once a service passes the initial assessment, they can get assigned control of the sub domain and point it at their own hosting.

GDS hosts a CF instance called PaaS (Platform as a Service) which hosts web applications on *.cloudapps.digital but the majority of the actual GOV.UK estate is hosted in AWS, as is GOV.UK Verify. I believe GOV.UK Pay might be on PaaS and a lot of internal tooling is.

(Source: have worked at GDS for the past couple of years.)

GOV.UK Pay use Amazon ECS, not CF. They do this because it is PCI compliant.

GOV.UK aren't quite yet entirely on AWS but are working toward that end. With GOV.UK, all the code is in the open where possible (https://github.com/alphagov/govuk-aws and https://github.com/alphagov/govuk-puppet respectively for the infrastructure).

Source: I used to work at GDS.

It depends on the project. CF isn't the only platform. I've worked on non-CF projects under the GDS umbrella. There is a bit of Azure PaaS and AWS too.

None afaik.