Social mistake

yeah they never do, the employer would likely keep it, and use it to reduce staffing costs. as a bonus for themselves the employer instead. thats how they do it in america i think in hopes that the employer can cover the staffs basic wage by the tips and so not have to dig into their actual revenue to pay the staff with.

anyway, in the uk we dont tip..... so the restaurants actually already price in any desired tip ontop of the price of the food and service you get... so if you pay a tip in the uk your actually paying it twice as the desired tip is silently been added onto the menu prices lol the menu prices cover food, product and service plus profit for business and any desired tip on top the end result is the total price of the product you pay for, so when you pay a tip in the uk your kinda paying twice.

yeah that would avoid the employer taking it themselves.... although may still get the waiter in trouble if they try to hide it and actually keep it themselves and get found with it.

thing is though, the tip goes not to the waitress but to the employer who then keeps it themselves to cut down the basic wage they already pay the waitress.... furthermore in the uk we dont tip, so restaurants actually include any desired tip in the price of the food anyway so the basic price your paying for the food includes the tip in the price as they mark it up with tip in mind from a mark up... 

this also is normal because in the uk we ask what a product costs we get a price and pay it. service is included in any price. nothing more is required, service and product is included in all prices in uk offerings. if they say it isnt enough or expect more then its up to them to raise their prices, but they cant do this after as then its false advertising of the price. the price they charge has to be the price advertised to you beforehand.

id ask for a refund and leave personally.

i never tip, i find it rude for waitresses to demand or ask or even suggest you need to pay a tip. id get offended if anyone gestured for a tip or even mentioned i need to pay a tip, i wouldn't pay it... id give a verbal tip and that would be that the waitress gets paid more than me and should be happy with their basic wage and stop thinking they are entitled to another persons money for nothing when they are being paid for their job. if they cant do their job politely and nicely to top quality on their basic wage then they should be fired and put on universal credit to give them a life lesson on being appreciative of what they have. if they spit in your food for not paying a tip they should be tossed in jail and never allowed to work in public services again.

tipping is toxic that it causes things like this and makes service actually worse. the absolute cheek of a person thinking they are entitled to a tip.

Something similar happened to me ages ago. Went to a Christmas meal with (former); charity shop volunteers and coworkers. Nobody told me that had to pay towards the meal. Thought the charity did this. Lucky I'd the right money with me.

Another occasion was when used to have an annual Christmas meal at a restaurant. Some people wanted two courses and some people only wanted three. A fellow runner had to divide the bill up and told us how much we had to pay. The leader said we will bring and share at her house. I don't run anymore. 

They're being dramatic. Nobody in their right minds would spit in your food for suggesting giving them a tip! Did they think the waitress was neurotypical or something?

So you don't have to keep overthinking it:

You don't discuss it when the wait staff are at the table because they might feel like you are assessing their service harshly practically to their face since they are literally standing there.

With that said the group leaving it to the last person to pay who they know is autistic and maybe not caught on to the ettiquette to bring it up when it should have been something discussed as a group prior to that point is their fault for being apathetic to it or cheapskates and trying to hope everyone just "conveniently" forgets to do it/ bring it up.

And honestly if you the last person are only just bringing it up at the very end, then the server knows the group was trying to cheap out, the group are just mad at you not because you made the server feel bad, as a server I'd be happy you actually even thought about it, but you exposed the rest of them for being the skinflints they are and that upset them.

(Context: Was waiting staff myself once, will always be autistic tho.) Oh and if the meal is already over it's too late to spit in someones food lol, and it is not as common as people think it is either, most waiting staff are reasonably dignified professionals even if horribly underpaid, the ones that do that are not the kind of people who deserve a tip anyway as it is petty behaviour, would violate virtually every health code and carries a risk of losing the job and also criminal prosecution.

Thank you very much for your reply. You took the time and I really appreciate it. You have helped me understand a lot. I am very grateful.

I did this one, not in front of the waitress admittedly but I did it and therefore I got my parents to explain it and I can give the answer!

As far as I'm aware, current formality dictates a 10% tip as default, especially if a larger group.

It is not considered good etiquette to discuss this in front of the waitress though, because that would be equivalent to people discussing how they think you've performed while in front of you while not including you. Think your friends turning to each other and going 'well they didn't do x y z but they did do this' about you while ignoring you. I suspect this is the bit that your friends severely objected to.

In moving forwards. I agree that many people don't quite understands what the whole 'not understanding social norms and queues' is. And it can be very hard to explain. I think the way I'd try to explain it is it's like how you might learn the rules of a particularly complicated language. While most people can intuitively pick these things up for autistic people it often needs explicitly explaining. I'd suggest using specific examples from your history.

My favourite for me is that as far as I'm concerned as a child getting changed for PE my classmates went from it being perfectly fine getting naked in front of each other to boys and girls being in separate rooms and you being supposed to hide as much of your body as you can while getting changed (which did and always has seemed unecessarily faffy and inefficient) effectively instantly. Of course rationally that is not what happened, I just missed all of the transition until people were yelling at me about it. 

And then a positive example of that for me was with hellos and goodbyes. My parents worked out that I didn't see the point/remember/know how. So they talked me through it and would remind and lead me through it repeatedly and help me develop scripts for it. And that meant that some of the time (not all of the time, if I have a goal in mind I often still forget) I was much better at managing the hellos and goodbyes and people got less affronted by me messing up. 

I hope some of that was helpful, I also find toilets great places to decompress, they're highly underrated quiet areas. I'd suggest finding even just one friend to whom you can explain the more intricate bits of autism of so that you have someone who has some idea of what's going on.