Understand OP that some organisations can be too hierarchical still when it comes to matters of basic human functioning like sleep - particularly large international/government ones.
But your workplace is the UK, and you have UK contractual hours.
Obviously I understand there is a career progression risk if you tell them to shove their "opportunity" to work for 65 hours instead of 37.5 (as travel time is certainly not a personal perk!)
So maybe focussing on what THEY are trying to achieve with it is the best angle? Your boss wants you there to demonstrate the high quality output of her department. She wants to look good compared to global peers who are also marketing her home country as a destination to other global powers (or similar - sounds like that's broadly your role, but I'm presuming to fill some gaps). Would you produce your best work:
A. Travelling without the ability to sleep flat, then walking straight into the room to showcase the best of what you've been working on all year while looking like you've spent the last day in a bomb shelter?
B: Presenting remotely so you are fresh & functioning & can respond at your best to ad hoc questions during the day. But miss out on the in-the-room body language & side conversations.
C: Overlook the picky elements of the travel policy which are in there to prevent staff from turning work trips into de facto holidays/family visits by tagging extra days onto business travel to their (typically) home country. And book you on a flight 24 hours earlier (at the expensive of your own weekend & family time) so you are time-adjusted, rested and firing on all cylinders at the event starts - so you can showcase the best of what her department has achieved all year.
D: Demonstrate that you are actually valued as an equal human being with equal needs to protect vital bodily functions and book you on the same class of travel as her [not going to happen].
In further support of C, some businesses have strict rules about the number of staff from senior leadership/a single department who can travel together (whether that is car, train, or plane), as a risk-management measure. Sending you on a separate plane from her means even if there is travel disruption, one of you is more likely to get there to present your material.
Personally, after a turn at a company with as shitty international travel policy due to cost management (even for the CEO, who had to do economy everywhere, including 16 hour flights) I wouldn't take a job that had a differential long-distance policy based on seniority. But as you say, you have a mortgage to pay & only you know what other opportunities there are in your industry & how much you can push back within your current organisation in the absence of another job offer on the table!
Good luck!