Yes, socialising is indeed part of the reason I query
starting the first vaccination at 12 weeks in this case

that means to do things officially this pup will be 15 weeks before being able to socialise with other dogs, and on any outside grass areas, which is too late for the socialising window, whatever the pros and cons are for earlier or later vaccinations the socialising window is far more important.
With the bugs and virus's which seem to be spreading throughout parts of the country at the moment I would not be happy about socialising any pup of mine with another dog or anywhere another dog may go until fully inoculated.
There are reports of parvo going around in some areas and in the Midlands one of my older adult pups was near death with a bug caught and apparently the surgery was full of sick dogs around that area, my girls grandmother was also extremely ill last year with something she also picked up, so the threats are out there and I certainly would not risk anything without full inoculations.
There have been a lot of starting at 10 weeks, but to me 12 weeks is very strange.