There are different systems in different parts of the country but you mentioned London so don't worry about other areas.
GCSEs are similar to a high school certificate but instead of a single certificate you take a GCSE in individual subjects. They are normally studied over 2 years and the way the government rate schools forces schools to favour sciences.
So the average student will study English language, English literature, maths, double or triple science, one humanity, usually RE, a modern foreign language and then they con usually pick an option or two.
Grades 9-4 are a 'level 2 qualification' grades 3-1 are a level 1 qualification. They are external exams taken in year 11. There are different exam boards but it means a child in London sits the same exam as one in Manchester.
After GCSEs students do a level 3 qualification (or they can work or take up an apprenticeship but most continue in education).
Level 3 qualifications taken in school or VI form colleges are usually A Levels, and most students take 3 subjects.
Or they may go to a college and take other courses, college courses include things like BTEC which is more practical than A Levels but will still get you to uni. Think of them as similar to TAFE.
Students who did not get a level 2 qualification usually go to college for a level 2 qualification and then stay on for a level 3 qualification.
You can ask a school to teach your child, 'out of cohort' and I would suggest your eldest starts in year 10 so he gets the full 2 years teaching for GCSEs.
An alternative might be a place in college, most college courses are for students 16 + but they can attend from 14. It depends how academic he is.
When it comes to Level 3 qualifications A Levels will get you into uni to do medicine, other level 3 qualifications will get you into uni but not for medicine.
It also depends on grades.
Your 12 year old would probably be year 8, but you might also want to consider him starting high school in year 7 where everyone is starting a new school.
Something to consider is will this be a permeant move? Will they need to move back to the Oz system? Where do they intend to go to uni?
This is a link for Australian teachers wanting to teach in the UK.
https://teachin.com.au/resource/differences-uk-and-australian-curriculum/