The WHO guidelines recommend antiretroviral therapy (ART) begins when CD4 cell counts reach less than 500 cells to reduce HIV transmission. In the UK, 96 000 people were living with HIV (PLWHIV) in 2011, ART coverage was 84% among the diagnosed population and 42% of PLWHIV had detectable viraemia. Using published methods, we estimate starting ART at below 500 CD4 cells could have reduced the proportion of PLWHIV with detectable viraemia from 42% to 38%, whereas halving the undiagnosed population could have led to a decrease to 28%. In the UK, it is unlikely early treatment will reduce HIV transmission unless the undiagnosed population is substantially reduced.