Lots of working class mums that are young and single, literally paid to breed and live off benefits, which increasingly is creating a big underclass. The richer are having fewer kids.
Since Britain is mostly without war, famine or disease, we need education to do the heavy lifting to control the population. In areas, social and economic circles where education is low, the population tends to go up.
A big part of why couples stay together are the pressures that keep them together - economic, social, societal, legal. Right now is just a really shitty time for married couples in Western countries - it is increasingly anti-social, anti-religious, much more individualistic, ultra mobile, but high on public services. Parental laws heavily favor mothers over fathers. So, combine situations where populations tend to go up with a legal / social environment where staying together is completely unnecessary for leading a meaningful life and we have a large amount of single mothers.
We can blame the single mothers, or we can assume that, given a set of opportunities and constraints, people generally behave in a predictable fashion. Don't hate the player, hate the game.
Incidentally, if you really think that Britain's burgeoning peasant underclass is the fault of self-impregnating single mothers you've been mislead; I suggest you change newspapers.
Having most of your kids being born to a society's most impoverished and least-educated members is not a recipe for long-term success. Saying this isn't blaming the poor, it's pointing out that there's something seriously wrong with the society, and it's not going to lead to the society improving in the long-term, but probably rather the opposite.