£2000 is less than a terms rent nearly everywhere! What do you think is affordable? Have you any idea of costs in 2026?
Yes. My youngest is still a student.
Also, the OP isn't expecting her to pay all the rent, she will have the minimum loan. This is about how they make up the difference between the min loan and the rent. Which is sadly normal for many (most?) families now accommodation is so much more expensive than it used to be 30 or even 20 years ago.
Did she know her parents didn’t have spare income?
How would I know ? 
But the fact it had all been agreed that the £2K she had earned last year was for University and agreed she was going to work during her gap year to earn, suggests she does realise she would need to contribute, yes.
Many dc work and then spend the money on a holiday.
Which is nice for those whose parents are able to fund their dc in full. But equally, there are also "many dc" who work to earn their own money to get them through University. Each family is different. It's not just about income, it is about all sorts of things which affect your budget - including being fair to other dc.
How will DD earn rent in a gap year? For how many years - all three? It’s just ridiculous.
Well, if she is only able to get a minimum wage job, and only worked for 12 months (rather than the 14 months she could between this year's A levels and going to University after a gap year), then she would earn £22,568. as an 18 year old. I don't know if she has skills to be able to earn more than min wage, which obviously she could. Obviously some tax, NI, and pension, and then living expenses to come out of that, but, living at home and not having commercial rent during that gap year would still leave her a really decent chunk of money to help out.