People began crossing the channel in 2014, six years before the UK left the EU. Source migration observatory.
The EU migrant crisis began in 2015, five years before the UK left the EU. An estimated 1.3 million fled troubles in the Middle East region to seek asylum in the EU.
Boat crossings had increased significantly by 2018, two years before the UK had left the EU. The cause is attributed to tightening security regards lorries and ferries which prompted migrants to attempt entry into the UK by crossing the channel. Source migration observatory.
In 2020, two million people sought asylum in Europe as troubles escalated in the Middle East. By 2022, the number was five million.
Also, for information, the number of people crossing the Mediterranean Sea is substantially higher than those crossing the channel and measured in the 100s of thousands as opposed to 10s of thousands crossing the channel.
Was Brexit the cause of the troubles in Africa that have prompted 100s of thousands to flee across the Mediterranean Sea? No is the answer.
As others have pointed out, there is a difference between causation and correlation. However, remain supporters rely on correlation in an attempt to prove that increased immigration in the UK is due to Brexit only.