How would the college know what paper to put me under etcThis wasn't very clear. Are you asking what would the offline college that you would be attending know which tier to put you under or are you asking if you were to sit as a private candidate which paper would put you under?
If it's the former, that would be down to the individual teacher teaching the GCSE at the college. They will have an idea of your capabilities and allocate you to the appropriate tier based on your performance throughout the course. Having said that, the default option would usually be higher tier for both, since you need the equivalent of an NVQ Level 2 in English Language and Maths in order to do any of their courses or NVQ Level 3 (this isn't a requirement set by the exam boards by the way). You usually have to get way more marks to get the same grade in a foundation paper as opposed to a higher paper, so in a sense it's "easier".
If it's the latter, you would have to book it yourself. If you sit as a private candidate, you're picking the paper that you want to sit. Typical steps to booking the exams as a private candidate (this includes A Levels as a private candidate, which I don't recommend unless you don't have other options):
1. Find the Private Candidate section of the exam board's website.
2. Find the list of approved exam centres for your exam boards
3. Liaise the exam officer for your chosen centres - you're going to likely need to contact 5-10 of these as some would take their time responding to you, should they respond at all
4. Book your exam and pay the exam and admin fees
If you have speaking, coursework, and practical assessments in your subjects, you would likely need to check to see if the exam centre concerned would be willing to accommodate these components of your assessment. If they don't, you would need to find another centre. Usually, these components that don't typically involve you sitting in an exam hall would likely involve a higher cost.
Exam fees for GCSEs are different to those for A Levels, so you would see a significant difference. However, these should be standardised based on the exam board.
The admin fees depend on the location of the individual exam centre. Those in cities tend to be more expensive.
For Steps 1 and 2 from above:
OCR: If you go to the OCR website (
https://www.ocr.org.uk/students/private-candidates/), they would point you to the following page:
https://www.jcq.org.uk/private-candidates/EdExcel: Similarly to OCR, if you go to the EdExcel website (
https://qualifications.pearson.com/en/support/support-topics/understanding-our-qualifications/where-can-i-take-edexcel-exams.html), they too will point you to:
https://www.jcq.org.uk/private-candidates/AQA:
https://www.aqa.org.uk/student-and-parent-support/private-candidates/finding-a-school-or-college