One of the times I went into a slow and busy M&S cafe, they wouldn't allow table reservations until you had your food.
A member of staff was keeping an eye on out for free tables, telling table baggers to wait their turn, escorting people with food to free tables and encouraging people who'd been in there a while and had finished, to move on so the table was freed up for people waiting.
Worked quite well and kept the tables available for people who actually had food ready to eat rather than those who were waiting to get it, or lingering after they'd eaten it.