The problem here as I see it is:
1) There is no spare time in your schedule. You are "incredibly busy", and you're already behind on your work (proven by the fact that 30 minutes would put you "even further behind"). But I assume you're not "catastrophically doomed to be massively late on everything you do", or you'd have mentioned it.
2) This extra presentation thing has come up, which (because your team is already stretched) your boss can't easily make time for. It will be at least once a month, quite possibly more.
If this "stretched" assumption is wrong, then the correct way forward is extremely simple: ask your boss to let you take your lunch break immediately after the presentation. If you're not already over-stretched, that's an easy "yes". If it's not an easy "yes", your boss has a resource problem, albeit a fairly mild one as these things go.
So, from the POV of getting all the work done in the available normal hours, your boss is screwed. Maybe it's their fault, maybe it's somebody else's fault. Doesn't really matter unless you have a realistic chance of getting up the chain and challenging whether this presentation and/or some other part of your work really needs doing at all.
If you can get rid of something from your already slightly over-committed schedule, obviously that's a good option. For example, if everyone is flat out all the time, that might be a good argument for making something more efficient, dropping the least valuable thing you do, or hiring another person. Frankly, if your boss could do this they probably already would have, but if you have the right kind of relationship then you can use this a spur to propose to your boss they give it another go. Let's assume it's not that easy.
Also bear in mind that people tend to over-estimate the risk of just asking. "So, boss, what's going to happen on the days of this lunch meeting? Did you intend me to take a break at a different time? I'm really not sure I can work through". It's plausible your boss literally hasn't even thought about this, and even though things are stretched doesn't really mind what you do at the scale of an occasional 30 minutes. Personally I'm used to offices where people can take their lunch at varying times as convenient, rather than the same fixed slot every day, and so a lunch meeting is in the eye of the beholder. You have a fixed lunch time, but it's possible your boss just assumed you'll figure it out.
If that fails, the question you ask yourself is, "am I going to help my employer out by giving them something for nothing?". If you are, then your course is pretty clear. Go to the meeting. Do all your normal work. Explain to your boss that you really feel you need a proper break (this is usually easy if your break has legal protection, but could be be hard if it doesn't because they might think listening to a presentation is like a break). Figure out with them when during the day you can take that break. Leave late on the days when this happens.
If you're not going to help your employer out for nothing, then they might be willing to accept that. You know your employer, so try to estimate the likelihood that they will pay you overtime and/or have you do a bit less "normal" work to make up for the lost time. If the likelihood is high, ask your boss for that. If the likelihood is low enough not to even try, or you ask and don't get, then you and your employer have incompatible goals and must negotiate to get anywhere.
If you're not willing to help your employer out, and they are not willing to accept that, then you are locked in a battle of tactics and will. There are a whole bunch of different options for different kinds of negotiation (or unilateral action in lieu of negotiation): stand on your legal rights (if you have any). Look for a better job. Go into a hard-ball negotiation with your boss, where you say "look, if you want me to go to that lunch meeting here's what has to happen". Try something a little sneaky, such as going to the meeting and then immediately afterwards say "OK, I'm just heading out for lunch, see you in 30", implicitly daring them to disagree, and let your normal work fall where it may. Be aware that locking yourself in a battle of tactics and will with your employer usually has big downside risk.
As in any negotiation, you need to know (a) what you want and the value to you; (b) so far as you can, what your opponent wants and the cost to you if they get it; (c) what your BATNA is (Best Alternative to a Negotiated Agreement -- look it up); (d) if possible, what your opponent's BATNA is; (e) the potential cost of even entering a negotiation at all, given that your boss might retaliate against your perceived trouble-making.
The fact is that unless you're on hourly wages with paid overtime, or otherwise have very strictly defined work hours, you are constantly in something of a negotiation with your employer over how much work (if any) you'll do beyond standard hours. Different people are in different places: some never have to work unpaid overtime at all. Some are happily working 50% or more over their nominal hours, every week, forever or until they burn out. Where are you willing to sit on that scale?
The fact you're asking this question suggests that currently you're pretty much working standard. But your company has some management or planning difficulty that means they're consistently over-committing and running behind. Be aware that this problem will not be solved by you working a little bit of extra time. There is no manager on the planet that can successfully and smoothly plan all work to get done on time if their team work 40.5 hour weeks, but cannot get it done in 40. It's always a trade-off within a continuum of risk. Furthermore, everyone would always prefer their employees to be busy than not. The problem is not the 30 minutes for the presentation, the problem is the consistent over-commit which leaves no flexibility for people to invent a 30-minute presentation. You doing them a favour by producing an extra 30 minutes out of nowhere is a temporary workaround, and it is well within what a lot of people expect in their job, but you have to judge whether or not that's all that's needed. You also have to judge whether or not this is the thin end of a nasty wedge.
There is no single action to solve all those issues, especially given that we don't even know your location or yout employment status. But all the above is just detail on top of the basic things you have to consider: "how bad really is this?", and "will my boss even do anything about it if I raise it?". Your best guesses to those questions are better than ours.