> On one end £9 of labour cost for a plate of asparagus seems deeply inefficient and unrealistic, particularly when the cost of ingredients that also include (hard) labour is £2.

Presumably the staffing cost is the front of house staff as well as the actual cooking and then the cost of employing someone to wash dishes, clean the restraunt and so on. Then compared to growing asparagus which seems to largely come from countries with substantially lower wages. Restraunts have always been infamously low margin businesses though.