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

Yes, this is generally how things work in my circles.

Separate bills is a complete pain for restaurants. Just sort this out yourself in advance.

Everyone who orders the most expensive thing, various cocktails etc thinks others won’t notice. They do ALL OF THE TIME and the offender is marked down as stingy and mean. Dont let that person be you.

If there’s a large group. Broken down his mixed grill is likely to cost less than a pound per person.

There will also be at least one person whose drink is more expensive. Or orders dessert. If someone has an extra Coke does it both you this much?

What if you want a meal at £10 and everyone else orders from the same section but theirs are all £13. They would be putting up the bill more than his mixed grill.

Do you have an issue with your Bil in general?

Well say you guys all share it out and I will pay my own.

Personally we always just split it when family, but it seems to really bother you, and also you may not have as much free money as the rest of the family so just say you can't afford to split and will just pay for what you have.

I absolutely refuse to be taken advantage of and pay for what I have plus service charge or tip.

We now pay what we roughly have had. Or one person pays then we work out what each person had and requests them to pay. We are a mixed group some drink, some don’t, people have 1 course v 3 & some can’t afford (just at end of maternity leave as an example) some just meet us for a couple of drinks and snack.

Since we have done this the night outs are easier as no one worry’s what to order or what the bill will be! And the ones that want a few cocktails can and pay for it while the person sat with a water doesn’t have too! They also better attended as everyone knows what to expect financially.

I think you should have what you want from the menu but I would offer to put more in to cover it or offer to cover the tip

In that case my experience is possibly not the norm, going out for a meal isn't a rare treat for me it's a regular way that I meet up with friends and family.

Like the OP I'm not talking about high end restaurants but the pubs and small restaurants round me seem to be busy every day of the week so I don't think I'm that unusual

Maybe it depends where you live

"I didn't eat any calamari so I'm not paying for calamari"

There isn’t, surely that was also evident by your BIL simply ordering what he would like to eat from the menu rather than from the reduced section of the menu you’ve decided in your own mind he’s ‘allowed’ to order from.

I think the difference is you see it as them trying to purposely shortchange you, whereas they’re just simply ordering what they fancy in a non malicious way and because nobody has suggested getting separate bills they’ll not give it a thought - why wouldn’t you always have ordered whatever drinks you fancy?

Incidentally OP how much is the difference in price normally, and how many people are at the meal?

oh you thought there was a rule........

I can’t imagine going for dinner and not ordering the meal I wanted to eat.

Of course it’s rude to order lots of extra sides or additional courses or drink far more alcohol than others if you are splitting equally. But to want to restrict the menu choices is joyless and stingy. And how much difference does one person in a group having the mixed grill make anyway? It can’t be much more than £15 more expensive than what you had OP. Split between everyone it’s almost nothing. You are totally unreasonable in my opinion. I can only think you just don’t like your BiL.

Because going out for a meal is a rare treat for most and it’s a social occasion- who knows maybe the BIL would be more than happy to pay for his own share but the mad family dynamics here mean that he’s not allowed? Or maybe he just loves mixed grills and thinks that because no one has said anything and they keep going to places with mixed grills and no one has actually SAID ANYTHING that no one has an issue?

Well you have got a few choices. You can stop going along with splitting the bill equally or stop splitting ther bill altogether, you can go off main menu yourself and see what happens, you can stop going, you can suck it up. If its an "unwritten rule" then how do you know about it and who set it? My answer to what i would do varies. With one group of work friends, we used to pay for what we had as we were all in different and variable financial positions. With others, where we were all in the same (better) financial position, we'd just have what we wanted and split the bill BUT even the "specials" weren't that much more than the standard. Does your bil try and weasel out of paying his share and is what he chooses very much more than what everyone else has....a main menu with sides can often add up to more than the special with less or no sides and adding in cheese or desert can bump the cost.

We go out for a meal with another couple occasionally and we always split the bill (I'd rather we just pay for ourselves but don't want to seem petty when the other 3 want to split). They always have a starter each, bottle of wine & obviously main course. As we've been short changed a few times in the past I now always have a starter when I didn't used to & whatever drinks I fancy! I can't imagine others having to subsidise if I have expensive taste. And others will always notice. Apart from my dh who likes to treat people 😕

The is no unwritten rule no.

I used to go for a pub lunch with a book group. They were all wealthy women apart from me. I was very surprised the first time that the Queen Bee requested separate bills. So we all went up separately afterwards to pay. All was itemised on the till so no hassle for anyone and no worrying about whether only Carol had had a pudding.
Conversely, meal out with the office where at the bar first, all the blokes were buying drinks and being generous. Then after the meal, splitting the bill, there was a big hooha about someone ordering garlic bread and not everyone eating it so shouldn't be paying for it.
I know which scenario I prefer. If it makes me look tight, then tough.

If course you order what you want but unless you're a selfish knob head you recognise that and chip in the extra, surely that goes without saying