I mean neither, as others say.
Neither degree leads directly to a high-paying career after all.
Are those the only two options under consideration? And is a high-paying job the only or main motivating factor?
As I understand it there are roles where you can earn lots of money and they tend to be financial, in London. It’s very much not my area tho (underpaid creative here) so I am not the best person to ask. Other options for a well-paid role include engineering and law. But there is a good deal of hard graft in all of these options.
She could find a graduate role with either degree – I agree with PPs she should try and get as much experience as she can to get her foot in the door.
Reading your updates - bio is the only science she is doing for A level? does she also study psych? That's often counted as a science (tho not everywhere it's true). If it's UCL she is fixed on as the top uni (?) then she needs to study what she can study there. But it's quite possible (more than possible) to get a great degree and a good graduate role with a degree from somewhere else.
Where else is on the list? It does sound a bit as tho you (or she - I hope she tbh) have thought "oooh UCL is the best, what shall I do there?" I agree with @titchy that London unis in general can be impersonal places. What kind of person is she?
Honestly tho – she needs to take the subject she will enjoy. Three years at uni is a lot of work and you need to love your subject. DD did a sloppy hums type of degree (so no work at all haha <joking>) and I was somewhat swept away by how much work she had to do and did (she was at home for a lot of it bc Covid which is why I know). Luckily she loved her subject.