Gov.UK is supposed to be a bit like BBC 1 or Radio 1 – in a national emergency they can be taken over to disseminate critical information, like if there was a nuclear attack launched on the UK.
A lot of dynamic sites use Fastly for its programmatic edge control and a near immediate ( ~1s-4s, typically around 2 ) global cache invalidation for any tagged objects with a single call to the tag. That feature alone simplifies backend logic significantly. To make this feature portable to CDNs that do not support it and provide only regular cache invalidation requires a complicated workflow setup which significantly increases the cache bust time, which in turn removes all the advantages of the treat dynamic content as static and cache bust on write approach.
I proposed and lead our multi-CDN project at Pinterest for both static and dynamic content and I can tell you, many many times over, it has been well worth the effort. Everybody should do this if not only for contract negotiating leverage.
Cache invalidation is fast enough on all CDNs now for most use cases (yes, including Akamai). But realistically, most sites (Pinterest included) are not using clever cache invalidation for dynamic content because it’s not worth the integration effort (and it’s very difficult to abstract for large 1k+ engineering teams). Most customers are just using DSAs for the L4/L5 benefits (both security and perf). In that case, it’s not complicated to implement multi-cdn.