This is terrible behaviour. They know exactly what they are doing and your boss is complicit.
Here on mumsnet, the forum users always want to make the OP the poster girl for whatever it is that they're too scared to do themselves. We'd all like to see you vanquish the bullies and fist-pump the air as we read about it, but having read your posts on the thread, I don't think that's the role for you.
If you go to HR, the atmosphere will become even more unbearable for you during the investigation and in the aftermath. Nasty women like that, who think their behaviour is justified will not change their mindset, even when their behaviour is made explicit to them and they are threatened with consequences; they will just take it underground and find more insidious ways to bully, which will be even more harmful to you.
These people are NOT the people you need to expend money and time eating with outside of work. They are draining your energy, harming your mental health and probably making you miserable at home. The best thing for you is to leave this toxic workplace.
There's a couple of things I feel you could do:
Apply for another job and when you get one (and not before references, contract etc), make a formal complaint to HR. The atmosphere will be very difficult for you, but you're helping other women in the workplace, so it's worth it. You can always go "sick" if the situation becomes untenable in those last few weeks.
If you cannot get another job, take the money you would spend on the meal and taxis each month out in cash, put it in a jar and when you have enough buy yourself something beautiful, like a good handbag, a ring, perfume. Something you can take to work and secretly enjoy, look at and smile to yourself while they're planning an evening out. Even better if one of them admires your purchase.
Or you could just get another job, leave their goodbye flowers and card on your desk, walk out and never give them another thought.