Given that it is the birthday of your GLW, and a family celebration of said event, I would find such financial detail nit-picking to be petty in the extreme.
Steam in, order whatever the fúck you want and say fúck all when the bill comes. Let them sort it out.
3-5 pints in advance will help in the event of any negotiations, trust me. No whiskey, or bugle.
But imagine how you'll feel? At your wife's lunch, a bottle or so of something to the good, picking over the bill with your glasses perched on the end of your nose like some sort of bean-counting cùnt?
Really? You might as well then drive to Beachy Head and do the decent thing imo.
A quick "Here's £x as I have ordered the most expensive food" or "let me get a round of drinks as I...." etc
Many years ago I went for lunch at The Savoy with my parents to celebrate their umpteenth wedding anniversary, along with my two sisters, their husbands and their children. Magnums of champagne were enjoyed by one and all. The bill was in four figures. I quietly settled it, expecting to square up with others later.
Both brothers-in-law approached me quietly afterwards, shook my hand and thanked me for the wonderful meal.
I feel like I've earnt the right, over the years, to nit-pick a bit now that I am entering my final couple of years of earning a living.