The reduced council tax isn't a benefit. Council tax is based on two adults living in a property, so a couple paying £1000 in council tax is paying less per person than a single person paying £750, i.e. the single homeowner pays more.
Couples don't have to have children, but if only couples who could easily afford bringing up a child without any issue of putting money aside for retirement and paying for all the other things that go with living, few people would have children, which would ultimately bring national problems of its own. We would then have to have immigrants to fill the workplaces and pay taxes to fund the social services which support the large bulge of elderly people which on a population scale have high healthcare costs. Still, that won't be a problem, the UK population loves immigrants, doesn't it.