An old Edexcel document with many classic investigations:
edexcel-gcse-coursework-tasks-and-projects2540-new-v2
Nrich – Investigations – articles and resources
Transum Investigations Resources
On Tes, Maths Investigations (targeted at Year 7), Mathemagic (KS3), and
100 Maths Investigation Ideas (90 page pdf) for able GCSE and Sixth Form students
From OCR: Maths investigations OCR
From Teachit Maths, Rich Tasks for KS3-KS4 Maths
On STEM Learning – Lorraine Mottershead’s book: Sources of Mathematical Discovery.