It's probably overkill, but not really enough overkill to be worth spending much time on.
E.g. 12 worker nodes and 192 vCores means they've picked 16 core nodes. 1.5TB of memory across 12 nodes means 128GB per node. 24TB of storage is just 2TB per node.
So it's 12 relatively mid sized servers/VMs.
They could certainly do it with much less, and I have no interest in looking up what 12 nodes of that spec would cost on Azure, but at Hetzner it'd cost less than 1500 GBP/month including substantial egress. At most cloud providers the bandwidth bill for this likely swamps the instance cost, and the developer cost to develop this is likely many times the lifetime projected hosting cost even with that much overkill.
If they happen to have someone familiar with query caching and CDNs, I'm sure they could cut it significantly very quickly, and even an entirely average developer could figure out how to trim that significantly over time. But even at (low) UK government contract rates it's not worth much time to try to trim a bill like that much vs. just picking whatever the developers who worked on it preferred.