to feel that DH's work should not expect him to get a passport?

DH is a TA in a special needs secondary school. The assistant head has asked him to go on a ski trip to France in January. Going on residential trips is not expected as part of the job role and is done by staff on a purely voluntary basis. He has already been away on residential trips in this country - something that many of the staff, particularly at his level, refuse to do on the grounds that they don't get paid enough. He tends to put himself forward for these trips as the students get so much out of them and without staff willing to go they wouldn't happen.

He doesn't have a passport, he has no need to go abroad and we aren't planning on taking any foreign holidays any time soon. When our passports expired we made the decision not to renew them as it seemed like a waste of money as we have no intentions of leaving the country.

DH has told the assistant head that he would be willing to go on the ski trip but can't because he doesn't have a passport. The assistant head is now pressuring him into getting a passport so he can go, as they have a lack of willing volunteers. DH is feeling like he is being guilted into doing it for the students and that he is letting the team down if he says no.

Am I being unreasonable to be a bit annoyed at the prospect of us forking out £75 for something that DH is not going to have a use for after the trip? Not to mention having to pay for appropriate clothing/gear which again, he will have no use for afterwards. He won't be doing any skiing as they have instructors for that but he's still going to be out on the slopes all day.

I've been saying that I think he should say no. We aren't very high earners and it will put a dent in our budget. He is on the fence about it. AIBU?