Obviously the value is something only you can comment on.

£400 one way is a tall order for WT to WT+ in terms of additional benefits. If there are Avios seats in Club and the fare class you book is upgradeable it'd make it more palatable as Y -> J is a much bigger difference.

As it's such a late departure, you might find it easier to doze off than if it were an earlier flight. If you get a window seat you've got something to rest against it could be not awful. A bulkhead could be useful for not having someone reclining into your space as well - unsure if you've got status that'd allow you to pick seats for free.