OP, I have a DD who studied Geog at Oxford. She had all 9s (but from a very selective school so contextualised as such). She didn't get in first time round, but reapplied with 4 A stars achieved and got in.
Was anyone at your DD's school getting lots of 9s? If not, and if 8s were outstanding in the context of her school, her GCSE grades should be fine. Everything is heavily contextualised.
Things your DD could start now -
Join the RGS (if in / near London, go to some lectures maybe)? Or watch online if possible?
Get the Royal Geog magazine subscription
Look at entering the RGS essay competition
Other essay competitions - eg. John Locke (doesn't have a distinct Geog category but there will be questions that can be answered with Geog perspectives / theory). I think Dukes Education (??) used to have an essay competition where there was one world like "Inequality" and then they could approach this from any discipline. Look also if any Oxford or Cambridge colleges have any relevant essay competitions.
Do an EPQ - does her 6th Form offer this?
There used to be an online student magazine for Geog called 'Routes' where students could enter articles and potentially have work published. Not sure if this is still running, but have a look.
Is she interested in physical or more human geography?
If Environmental issues are her interest, could she volunteer in any projects locally? Or if she's interested in human geography, she should start to think about her local environment through this type of lens.
If she wants to do some intro degree-level thinking (relevant to they way current teaching at these unis) she could look at perspectives in 'more than human' geography that basically examine how space is experienced by other species - how we can measure this and what are the confluence of cultural, economic, historic, political factors that bring this about - basically the 'biopolitics' of the what and the where of what allowed to live and what is not. This is a big theme in Geog at those unis (maybe a basic intro to Foucault and drop his later ideas around biopolitics into the PS - eg. how this has help her to think about her local environment differently). Also the whole issue of the Anthropocene (human behaviour is now the greatest impact on global systems) - ie. a new 'era' is upon us (there is disagreement about this). Or maybe it's a new 'epoch' that is the debate? Sorry, I didn't do the degree; I just picked this up from listening to DD and friends. A Geog degree is extremely diverse and not what people might expect.
Obviously she doesn't need to do all this!!! It's just a few ideas and she might use one or two for the PS.
Good luck to her!