According to David Willetts, President of the resolution Foundation and a Member of the House of Lords, pre Brexit, there were no illegal boat immigrants.

“There were no small boat crossings bringing in migrants until Brexit. They began in 2018. In the words of a study by Professor Thom Brooks of the trends “There are no records of any individuals travelling by small boat to claim asylum in 2017 or before.” Before Brexit illegal migrants could be and some were returned to other member states of the EU. That was how the migrant camp at Calais was closed. But the returns policy was abandoned as part of the hard Brexit deal.
Brexit also meant we lost access to data in other EU member states about criminal records of migrants coming to the UK. We have also lost access to the EU fingerprint data-based of migrants so tracking their history is more difficult.
Brexit was based on a promise to bring back control. Instead it has weakened our control of our own borders by ending the collaboration with our neighbours. The populists brought us Brexit with promises of taking back control, but it has had the opposite effect. So Farage helped create the conditions in which small boats come here, and now blames the out-of-touch political class for the consequences of a populist cause he promoted.”
It all seems to follow things we know about what happened. Due to the appalling decisions to drop the returns policy, stopping collaboration with our EU neighbours on migrants, dropping access to the fingerprint database, all part of the awful Brexit deal, the UK now has the "boat people problem".

Ergo, started with the lies of the Brexiteers and compounded by Johnson making stupid decisions with regard to the deal he "negotiated". Oven ready deal my arse.

I wonder how they'll respond to this in the Blue Tories and Reform?