Former GDSer here[0] - this was one of the things that really grabbed me was the focus on design for users -- and this included release cycle as you say!
In addition, we were encouraged to throw away PoCs (that had passed the first service assessment) and start anew to make sure we ended up with a great service at the end of it. This meant that inside a pretty large organisation, testing and validating new services was relatively easy as you could cut corners on the PoC and have an institutional understanding that you'd probably throw it away entirely and start again with something more solid.
One of the mantras that was oft repeated was that 'people do not have a choice to use our service' -- because it was a service that only the govt could provide for whatever reason. The government service manual is amazing, alongside things like the service design rules (more aimed at UI/UX designers).
In addition, the entire team for a service or section thereof would participate in nearly everything. User research (actually watching users use the site) - everyone was there, from user researchers to product managers to developers and designers.
It's a shame more of it wasn't adopted by other govt dept (there are notable exceptions who have done a very good job)
[0] Plug - I worked on the new data.gov.uk
[1] https://www.gov.uk/service-manual