@neil
"I imagine that, in reality, “digital sovereignty” would be a remarkably expensive undertaking. Perhaps more expensive than buying commodity services from overseas third parties."
For business, yes maybe.
For the public sector, I suspect it's only expensive from a purchasing perspective. Yes, MS employ quite a few people in the UK, but a chunk of the £9Bn UKGov just signed will go direct to Ireland/tax havens. That £9Bn spent with Collabora/Canonical/etc would employ Brits, develop/support a UK skillbase AND mostly circulate back to HMRC eventually.
IIRC there was a report last year that EU industry sends ~€100/mo/worker to the US in software licensing... It's not just that the money could be spent domestically... by leaving the country, it doesn't loop back as tax revenue.
In that respect, "cheap" overseas vendors are only cheap on the price tag - they're expensive both socially and economically, but procurement doesn't seem to account for that monetary cycle.