Nah, the British are just delaying the doses, without any kind of trial in play (other than population-level observation).

There has been a study now, it's just not gone through peer review yet.

https://www.ft.com/content/de00fe75-9ba1-4be7-8cf1-6e9fa55f0...

Additionally the advice to delay the second dose is now also part of the WHO recommendations.

Note that this is the AZ vaccine. They're also stretching second doses of the Moderna vaccine, which is what we were talking about trialing here.