Prophead you are correct that in BA the FO can reject a take off, but only in a limited number of clearly defined circumstances. As most short haul aircraft generally have the same Vr and V1 theres little chance of a rejection beyond V1 as the first call will be 'rotate'. Furthermore company SOPs are such that nobody will have their hands anywhere near the throttles beyond V1 so it'll take a conscious effort to reject beyond V1. If anything I think your policy of allowing only captains to guard the thrust levers on take off is more likely to cause the confusion you speak of as it does give the captain the authority to override a 'stop' decision, leading to the confusion you want to avoid. By our SOPs it would take a quite remarkable amount of confusion to reject the reject by overriding the autobrakes and reselecting TOGA after the FO has brought the throttles to idle!