I appreciate your OP crabapple but most people are 'graspy' whether teachers or not, if we ran a ballot amongst teachers at our school regarding presents, most would fall into the "we don't want them, but will happily accept them and then slag the off amongst ourselves' category.
As a class rep I vowed never to do a 'staff gift' collection. Each parent can do what they like. Some years I given a gift, some years I haven't, but I've always said thank you because my son's education thus far has been good enough.
I HATE the pressure of organised gift giving (you must give a tenner, the teacher has seen a nice coat in Next) but I do think the small (usually horrible) little gestures MEAN more... I bought some compact mirrors on a 3 for £10 offer, they're lovely, there's not enough women in my family to give 3 away so teacher is getting one... a gesture of thanks, never to be repeated as I wouldn't go out and specifically pick a gift. Next year, well maybe there'll be another 3 4 2 offer that leaves something 'spare', maybe not, but there's always a thank you.
So really, the shit a teacher receives is worth more (and less) than anything else. Be thankful you're appreciated if you get nothing, a small homemade papier mache lighthouse, or some chocolates you can rewrap and give away.... as for £500 in vouchers.... pfft, fuck off. You give me a voucher for Next for being a decent parent, I'll give you a voucher for a paid job well done.
Its not unappreciative. crabapple YANBU. But neither am I to occasionally want to gift you... but if you want a diamond rather than a box of celebrations... well speak to the crazy 'collection freaks' and tell them you'll be happy with nowt. You'll be happy and so will I!