I always check the business class prices, and if the differential is not that great on a long haul overnight flight I'll take the business class seat.

I've also split it, as an example I've gone to LHR in J to get the lie flat bed to sleep in, and return in Y when it's a daytime flight.

As others have mentioned I've taken advantage of the BA sales this year when a J flight to LHR was under $2K all in. I'd be crazy to pay ~$1500 for Y when it didn't cost much more to go in J.