You're in luck, as I have all the past papers, mark schemes and examiner reports for GCSE and IGCSE Maths as well as A-Level Maths and Further Maths. I can send them to you, if you PM me.
I won't be too difficult with the resources online, and also some YouTube channels presenting the entire topics with breakdown in understanding proving vital to one's success.
Pearson Edexcel is the most popular board. You should be able to get a 7 with ease, more likely 8 or 9.
Then it is up to you whether you want to do A-Level Maths, Further Maths and IT!!!
lol
Here is a centre which accepts Private Candidates or otherwise known as External Candidates:
Campbell Harris College in Kensington, London is a short walk from High Street Kensington tube station.
Photo ID document (passport, driving licence or national photo ID card)
Passport-size photograph
Fees payable by card (not American Express) or cash.
Drop-in time for exam registration is between 1.00pm – 2.00pm Monday to Friday.
The exams are held at a huge hall:
St Mary Abbots Centre
Vicarage Gate
London W8 4HN
This venue is approximately 10 minutes’ walk from Campbell Harris (off Kensington Church Street).
https://www.campbellharris.co.uk/how-to-register