If you're sharing the cost of the meal there's certain things you don't go for

Why? If you’re going out to eat, why should you restrict your choice based on what other people order? You could argue that the others are being stingy!
If you're not happy with the cost differences, don’t split the bill - it’s easy!

It depends. If I really wanted something particular that was expensive I'd order it and then pay more to cover it at the end. I can't really think of a time that that's happened in real life though as generally when I go out (usually with friends not family tbf) we all have similar so splitting is fair. I think in your BILs situation it's cheeky.

I do have a couple of in laws who will order fillet steak if we go out with my husbands family and they know my FIL will offer to pay which I think is really cheeky as they wouldn't order it if they were paying themselves

Exactly this. If I'm out for a (rare) dinner I'll order exactly what I want to eat and drink, fully prepared to pay for it. Often when out with friends we'll work out the general split and then adjust up and down for those who had steaks / cocktails / no alcohol / no starter etc.

I also think it’s fine to order what you want but if I’ve ordered something particularly more expensive than other people I offer to put more money in. Often though it levels out as I’ve not had pudding etc

Don’t split - haven’t in decades… everyone pays for what they ordered. Dont want to be a chump or be stifled by others

I think people should order what they want as long as there’s not more than say a £10 difference. That said if I’d ordered something noticeably more expensive I’d probably just offer to stick a bit more money in at the time when we pay.

It’s a bit petty to quibble about main meal prices- unless the place is ridiculous then they’ll all be much of a muchness. And are you policing how many glasses of wine each person has, who has coffee etc (if your answer is yes then I think you might be better off just paying for your own stuff).

Well, I wouldn't split the meal like that tbh, it's always going to cause issues like "well why I am paying for X's expensive wine and steak when I ate salad and drank water all evening"

If you can't afford to split a meal where people are going to order what they want (I know lots of people who order steak when out because nothing beats a well cooked steak when out) then don't agree to split the bill

But people shouldn't feel they can't enjoy the meal because you'll judge them.

I wouldn't begrudge someone ordering a more expensive dish, but good etiquette would be for that person to offer to make up the difference if it is significant.

If it is someone who you go out with regularly, it might even itself out when you fancy something pricier.

Or, if this is trend, just say you'll prefer to split according to what was eaten.

In my circle, we order what we want. If someone had a steak, that person would probably put a bit more than the others, same with someone who had had much more alcohol.

I agree OP - I would find this a bit inconsiderate. Maybe suggest that you stop splitting the bill in future?

I order what I want and expect others to do the same I do t drink but don’t care if others drink good wine