sevenoaks lower 6th HL geography

Just realised you might mean Lower sixth form yes?
And higher level geography?

It really depends upon what exam board your sixth form is using, I don't know whether the curriculum has changed much (although I doubt it has much because it's geography), but my sixth form used the AQA exam board and we had these textbooks written by Barker, Redfern and Skinner, who actually write the exams.

https://www.amazon.co.uk/AQA-AS-Geog...a+geography+as - This was the AS/year 12 one that I used, they're really cheap on Amazon.

https://www.amazon.co.uk/AQA-A2-Geog...X6BH8NRSBYMYBP - And this is the A2/year 13 one.

I think the new AQA syllabus is split between Human and Physical geography textbooks now which makes more sense. But I doubt the subject matter will have changed much, it's usally: Rivers, Coasts, Plate tectonics, Desert/Cold environment, Conflict, Climate change and weather, World Cities, Health, Development and globalisation, population change, food supply, energy issues and to top it all off you had geographical skills (fieldwork and research skills that usually involve a trip to a river plus group projects and correlations). Most teachers teach the core modules which for physical is rivers and human it's population change. Then they pick whatever other physical and human topics take their fancy.
I studied plate tectonics, weather, conflict and energy issues as my optionals, but if you don't like it you could independantly study a different one (talk with your teacher about it first though) and when you sit the exam just write in the section you've learnt about.

These seem to be Redfern's new textbooks for AQA - https://www.amazon.co.uk/AQA-level-G...a+geography+as - This is the physical one

https://www.amazon.co.uk/AQA-Level-G...a+geography+as - And this is the human one.

Even if your sixth form uses a different exam board, the content shouldn't be too different at all, so I wouldn't worry if you don't need these textbooks as they're still useful and relevant for your studies, heck I used every textbook I could get my hands on last year. You'll be fine dude I hope this helps!