I would be interested to hear your source for these assumptions, and I do hope that you know that Caribbeans are Christian by default.
Not only do those that attend church on Sunday go "all dressed up" but they will spend the entire day there in multi hour services, have bible sessions on weekdays, and give 10% of their income to the church. Within this section of the community, they very much frown on sex before marriage as well as separation and divorce.
There are of course those that don't attend church with different values.
Just as we are discussing underperformance of white English working class boys as opposed to "white English boys" because white English boys from higher socio-economic classes are flourishing, we cannot describe Afro-Caribbean British families as a monolith.
Within the Afro-Caribbean British community there are working class, middle class, religious, atheist, 1st generation, 3rd generation, city dwellers, suburban dwellers - as with any community.
Specificity is a requirement in conversations like these, if the goal is to better understand the phenomenon as opposed to therapeutic venting.