Firstly congratulations to your Nephew on their degree
Just spoke to Marina on this and she simply said "A GP cannot refuse to transfer your records.". She said if this happens the first port of call is the Surgery Manager.
Her experience on the sequence of events is that you register with a new Practice, you are then allocated a GP. It is the new Practice that then request the records from the old Practice, not the GP as they do not own them.
It's worth noting that there are instances when a person doesn't know or cannot remember their previous GP or Practice and these people can still register with a new Practice and subsequent GP. All the medical history info in the UK is held electronically on what is called "System 1" and is accessible by all GP's on line using your NHS number.