MAD-LHR, it's load dependent.
LHR-MAD, BA opens up the cabin 72 hours before departure, and FLY decides whether you're suitable enough to have access to it. Approx 48 hours out, if the main Y cabin isn't too busy, the Y+ cabin will be blocked for trim.
Re the BA460/461 777 service, the WT+ cabin follows the same T-72 logic, though sometimes is not displayed on ba.com at all even at OLCI.