In which cases does it make sense for a company to operate its own company pension fund?

The main factor in determining whether it is worth a company running its own pension fund is the number of members. Since occupational pensions were introduced in 1985, regulations have tightened significantly and this increased level of complexity has led to higher administration costs. 

That is why more and more companies are opting to join a collective fund. In contrast to smaller pension funds, larger funds benefit from being able to spread their fixed administration costs across a large number of members. This means that administration costs per person do not increase or they only rise minimally.