I don't think anyone arriving newly in any country assumes the laws are the same as their mother country. What a load of rubbish.The problem the UK has is that it is a multicultural society and in many cultures, interaction between adults and children is legal, in their culture but their culture clashes with UK law because whilst the legal age for consent to sex is 16, and it is a criminal offence for anyone to be involved in a sexual act (sexual intercourse, sexual touching, kissing etc) with anyone under the age of 13 whether the young person agrees or not, on the basis that anyone under 13 lacks the capacity to give valid consent to any sexual act. There are cultures that legally allow sexual acts to take place with children under the age of 16 but as you can see, the UK has laws that prevent this so whilst someone may think they are behaving legally, whilst they are in the UK, what they are doing is actually illegal and as such the UK government has to protect children in the UK from all angles, culturally legal and criminal.
This wiki page shows the ages of sex consent of countries around the world and I think you'll be surprised at how young some of the ages are https://en.wikipedia.org/wiki/Ages_of_consent_by_country
Yes people living and working in the UK need to abide by it's laws but you can see from the wiki page, if you have people from many of those countries who have been brought up with their own laws on age of consent and interaction with children, it can be difficult for them to adjust to the laws of the country they now reside in. I'm not making excuses for them but I can understand the difficulties. Hence why UK children need to be protected. Just because it was OK in their country does not mean it is OK in the UK.
Apple communication safety feature is a very good thing for the UK. I just hope UK parents take it onboard.
Also, why does it sound like you're making it out to be only immigrants who are the perpetrators of this stuff? Laws are laws, and concerning children are strict and obvious in all but a small handful of countries, and even in that small handful of countries children are not a free for all, without any consequence. The UK has more than enough homegrown weirdos without trying to distinguish between those and immigrants.
What you're saying amounts to: "Immigrants are the problem because they dont understand that although they can have sex with children in their own country they come to the UK and do it here because they don't know the law". Faulty assumptions, faulty logic, and faulty premise.