Neither L:BC nor Capital had antennas David; they were simply programme contractors, whose output was transmitted by the IBA, in the same way that ITV programme contractors operated. Eventually, the Authority lost its monopoly over transmission and the programme companies became licensees, and were allowed to make their own transmission arrangements. (These have since mostly been bought out by a succession of tx cos, now known as Arqiva. )

But the first programme was LBC, on 417m, 719kHz, with Capital following a week later on 539m, or 557kHz. The BBC also rented the 417m facility for a while for the Radio 4 London MW relay, after the 1978 frequency changes resulted in Radio 4's new LW service, originally on 200KHz, suffering interference in Westminster. The MPs desperately needed to hear 'Today in Parliament', a MW/LW programme only.

But IBA holding the transmission licence aside, it was LBC, Capital, BBC Radio 4, and then the most heinous transmission of all, the Radio Authority snaffled it for jamming Radio Caroline, by adding a new ILR on 558kHz. A disgraceful move but it meant that the site had returned to the two original dial spots (in metres, anyway).
Radio 4 was shuffled off to the Crystal Palace site in Summer 2001, and the old power station is now luxury flats and shops