There are nearly three million people on the electoral role who were not there in 2016.
It just doesn't stand up to critical examination.

Peter Kellner, YouGov’s former president states: 395,000 reach voting age each year.( Assuming 3 full years- to be generous) That equals 1,185,000.It also assumes they are registered on the electoral role.

Government Statistics:
The total number of UK Parliamentary electors increased by 380,000 (0.8%) between December 2016 and December 2017, a smaller increase than in the previous year. This follows an increase of around 1 million the previous year.
The total number of UK Parliamentary electors in December 2017 was 46,148,000. The electoral roll does not directly correlate with those of voting age. Not every adult qualifies, those that are have to ensure they are registered.