Does ITV obtain its revenue from the British taxpayer? I believe that ITV has commercials and BBC does not, though not being a Brit I am certainly less informed on that topic than you are. An audience is captive when it is forced to pay for the service received. ITV is a free market service, unless I misunderstand. Nobody is compelled to support it.
No, ITV’s revenue comes entirely from advertising. Nonetheless, a TV licence is required to watch it, as it is for all channels. Until the many other digital, cable etc channels appeared in the last decade or so, the BBC and ITV pursued a relentless ratings war, which ITV won more often that not. This was not competition in an uncontrolled market, but competition it undoubtedly was. For the BBC, success in the ratings war was vital if it were to preserve its privileged status in the next Royal Charter/Licence Fee cycle. The fact that viewers in their millions chose so often to switch to ‘the other side’, as it was then called, shows that they were anything but an audience captive to the BBC: although the market available to them was, of course, limited.