With-profits annuities are linked to the performance of an insurance company's funds, while unit-linked annuities are linked to the funds in which your pension invests.
The unitised version was somewhat less opaque than the conventional version, with less surplus being held back, and also made possible switching between with-profits and unit-linked funds.
However, since the late 1970s the insurers have tried to compete directly with the unit trust market in offering a wide choice of unit-linked investment funds.