It seems harsh in the extreme to pin this on Cameron. No doubt employers will twist laws whichever way they can to suit them best. Case in point: before the national living wage was introduced, wages for the lowest-paid were so low that the government had to subsidise these low wages, eg by Working Tax Credits. Now the burden is shifting to the employers and while, yes, that's going to lead to a mess, it doesn't invalidate the concept of lifting wages high enough that people can actually live on them.
This is a problem for B&Q.