The seat you'll get on the BA A350s is frankly vastly superior to the LH/LX J seats which are, frankly, a hodge podge of outdated products.

The cabin IS big - but there is a smaller rear mini cabin with just three rows if you want to be removed from the rest of the dormitory!

BA is still serving meals on a one-tray service I believe so that's currently one other big differentiator vs LX at least, which did a proper service on my flight to Dubai the other week. But that may change by the time you fly anyway (frankly it should have changed already...)