My take on this:

1) stop being a tax haven. then fewer people would try to get to you and the world would become a better place 2) the UK doesn't even have that many refugees compared to many mainland countries - especially per capita and per GDP. for some reason many Brits claim that all refugees only want to enter your country 3) France suggested an centre to apply for asylum at Calais. The UK refused. 4) The UK has made it harder to apply for asylum from abroad, thus pushing more people to use boats 5) the UK is attracting some people to enter the country illegally due to the UK not having a national ID card, making it easier to live there secretly. How is that the mainland's fault? 6) the UK has withdrawn from the Dublin Agreement due to Brexit 7) Sunak might be calculating that judges will call him back on leaving the ECHR. Not only could it break the trade agreement with the EU, but the good friday agreement wouldn't allow that. Sunak must know this.