This page has links to A Level past papers and soluions. STEP and Oxford Admissions test links are at the end of the post.
Exam Solutions has thousands of worked video solutions to A Level questions. Note the tabs at the top of the site, select A-Level Maths and then your chosen exam board.
From David Smith:
OCR (MEI) C1, C2 & S1 papers and solutions
MEI Revision Videos – Further Mathematics Support Programme
AQA – questions by topic – The Chalkface
For an additional source of questions and full solutions for the Pure Maths content of A Level exams see MadAsMaths.
For an extensive collection of Challenging questions for Advanced Level students we can turn to the Oxford Maths Admissions Test, note the menu on the right hand side of the page, many papers and also solutions are available.