Assuming you're correct about the law, which I am not in any position to assess, the question in effect is, "how do I defend my legal right against my employer in this situation?". You should of course get some kind of individual advice to ensure that your reading of the law is correct and applicable to your case, before doing anything drastic. But as you present it this seems pretty simple:
- the law entitles you to the employer's choice of either a 30 minute unpaid break, or a 30 minute paid break in which you must remain in the office.
- on days where you boss requires you to attend a lunch-time meeting you are not getting the break to which you are legally entitled.
- your company is arranging important meetings to discuss projects, but not recognising that this is work and must be paid.
So something has to give there. First, appeal to your employer to give you your legally-mandated 30 minute break. Just approach your boss describing the problem and offering what you see as the solution. "There's an important lunch-time meeting you want me to attend, but accounts have told me they're optional and I shouldn't charge them. I'll attend meetings where they're needed for my work, but I need to get paid and I need the legal-minimum 30 minute break. What time can I take my break, and what can we say to accounts to sort this out?"
If that doesn't work decide whether to raise a formal grievance within the company. Somewhere along this process, they will explain to you how it is that in their understanding they are not breaking the law, which will help you decide how to respond.
The reason grievances often work is that a grown-up: someone in HR or senior management, can decide whether your boss is being unreasonable. If account are saying the meetings are optional and your boss is saying they are not, then it may be that HR or senior management will agree with accounts. Often they back managers, of course, there must be a reason they made them managers. But once they consider the legal situation there's a reasonable chance they will overrule one manager in preference to setting themselves up to lose a future lawsuit. Internal complaints needn't necessarily be acrimonious: if your boss wasn't aware of the law, and accepts correction, and you refrain from gloating, then it might be over and behind you fairly quickly.
If that doesn't work decide whether to take it up with the authority responsible for enforcing labour laws (who will also tell you pretty quickly if you're wrong about the law).
I don't think anyone here can tell you whether or not you should back down and accept unlawful working conditions, neither can they tell you whether or not you should quit the job rather than deal with the hassle. We don't know how important this job is to you, but it sounds like your break is important to you and so you probably should not just accept it.
Furthermore you need to be prepared to deal with a certain amount of run-around that might happen:
Maybe your boss tells you this is obligatory, but the accounts department says it's optional and won't let you record it as time worked. You have to push through this one: start by telling the accounts department that although it may be optional for many people, in your particular case it has been made non-optional and therefore they have to pay you. If they refuse to pay you then go back to your boss and say the accounts department won't let him make this obligatory for you and so you will not attend. It's hard to call this a "win", but once your boss and accounts are directly disagreeing with each other you can jump out of the way and let them reach a decision between the two of them as to whether or not your boss has the authority to require you to attend the meeting.
Maybe your employer says that the meeting is a break (with boss and accounts in agreement). Then by the law they still have to pay you for it since it occurs in the office, which they aren't doing and you can complain about that. And furthermore you can complain that anyway it's not a break, it's a meeting, so you still are not getting your break. You can then take that to grievance/authorities if necessary.
The most reasonable solution all around would be if they require you to attend the meeting, pay you for doing so, and then they also give you a 30 minute unpaid break at some other time on those days. If they won't see reason then your basic options are (a) suck it up and tolerate unlawful working conditions; (b) leave; (c) report it. (c) often leads eventually to (b), especially if your managers are not inclined to be philosophical about losing the odd legal battle.
It's all true that "company culture" may be that employees waive these entitlements, but labour law is there for a reason, and part of the reason is that employers benefit from (intentionally or negligently) fostering a "company culture" of letting them walk all over you. One sign of a good employer in this respect, is when management are going to staff and instructing them, "take your breaks, take your PTO, and report all hours worked no matter what".
What you have is different in magnitude, but not in kind, from "company culture" dictating that the one pregnant woman in the workplace waive her maternity leave entitlements, or that the one person in the workplace who gets sick waive their sick pay entitlements. The fact that other people don't feel the need for a break doesn't really signify except in that it leaves you as the muggins who has to decide whether it's worth the effort to fight your employer's crooked practices. Ultimately, even some murderers do get away with it, never mind workplace legislation violations...
Final thing, and this can be legally complicated, but if people are holding project-related meetings "of their own free will" in time the company doesn't let them record, then in certain circumstances under-reporting of hours worked might be a specific offence or might constitute fraud. This comes up most commonly on government contracts, but there may be other contracts where the client has stipulated accurate time reporting, not merely that all hours billed were actually worked.