So I agree with the other answers, that you are not expected to cover your own expenses. I would actually include the visa in that, it's no less a work related cost. And yes, you should tell the organiser that your institution will not cover this and ask if they will, but if they won't I think you can counter with an offer to present over video.
It's not uncommon for plenary speakers to present over video, even when funds would be available to fly them there in person. I've seen this at a few conferences pre-pandemic too. You are probably quite busy after all, and it's much less time consuming to present over video. Offering to present over video makes it clear you are not snubbing them, you really would like to share your work, you just don't have funds available to get there in person.
You could phrase it like;
I'm looking forward to presenting at "conference name", many thanks for
Looking at the logistics, is there funding available to cover my
travel expenses? These would be for the flights, accommodation and
visa. I estimate about (£XXX). Unfortunately my institution cannot
cover this at present.
If not, I'd still really like to participate, could I join you over
Thanks again, "dusa"
If they tell you that there is no funding available, or ask you to put in an application for something yourself, then there is a non-zero chance that they are predatory. You probably want to check. It's also possible that they are just a poorly organised student conference too, those certainly exist, so it's not a write-off. You might still want to offer a video presentation.
Best of luck.