Pre-COVID, IB generally had 1 widebody (the current afternoon/evening rotation) and BA generally had 2: the first rotation and the early or mid afternoon departure from LHR (varied by season, but either 460/461 or 462/463). BA generally had 767s on both, but sometimes one or the other was operated by a 777.

Since COVID, IB routinely has 2 widebodies (the second, I believe, flight out of MAD, returning late morning). BA no longer has 767s, and the longhaul fleet (well, all fleets really) is stretched to its limits. I'd assume that an A330 has slightly more cargo capacity than a 767, so we're talking about only a small loss of capacity. In the meantime, Brexit happened and the amount of cargo transported on the route took a dive.

IB isn't exactly full of spare capacity either, but with a banked hub, is able to spare their widebodies for a couple of hours between banks. The morning rotation generally arrives from the Americas around 6am and departs to LHR around 8am, and arrives back at MAD just after 2pm, in time for the smallish bank of longhaul 4pm departures. The afternoon rotation generally arrives from the Americas around 1pm, departs to LHR around 15:45, and is back just after 10pm ready for the midnight departure bank.