The main issue for BA here is the cabin configurations on their wide bodies are pretty premium heavy, which means that they really don’t have huge capacities. People going for Palma aren’t going to pay a huge premium for a flat bed, as they may do for a 12 hour flight to Singapore.
Even TUI normally remove the premium economy cabins from their dedicated short haul 787s for the summer, with other flights being operated between long haul rotations. But there is a huge difference in seat coats between an all economy layout for a wide body on short haul and one configured for long haul flights.
BA’s A320s seat a maximum of 177 or 180 passengers, and the A321s seat up to 218 or 220, obviously with big Club cabins those numbers reduce substantially.
In BA configuration the 787-8 seats only 194, with 60 being premium seats and 154 economy. The 787-9 has 89 premium seats and only 127 economy. Neither of those would justify the massive hike in operating costs on a short haul route. By comparison TUI’s 787-8 seats 300, or 325 without a premium cabin, and the 787-9 seats 345.
I think the only BA layout that could ever make sense would be the new 3 Class LGW 777... C32W48Y252. It’s likely that the Club cabin would stretch into WTP leaving 252 economy seats. But you’d have to weigh that capacity against the huge hike in fuel, handling, landing, ATC and crew costs, no point putting on a much bigger aircraft to fill it with £39 fares. In more recent years in Europe most airlines have preferred to send 2 narrowbodies for peak capacity rather than have the relative inflexibility of short haul widebodies that they will struggle to fill in winter.