"Approximately 1 million a year net" is completely true. In 2023 the UK's net migration was 906,000. But for context I'll add that it was a historic high and in 2024 net migration fell to 431,000.

The Migration Observatory at Oxford publishes excellent summaries about migration trends, e.g. https://migrationobservatory.ox.ac.uk/resources/briefings/lo...

According to wikipedia for causes of vote in favor of brexit, one third said immigration.