A free (open-source) Psychology textbook (plus resources) that 
could be used to supplement your existing textbooks and classroom resources.
As with the open source “Introduction to Sociology” textbook, “Psychology” is a free open-source textbook, now in its 2nd edition, created and distributed by the OpenStax College. It comes in a range of versions, most of them digital (although there’s also an option to by a paper version of the text for around £20 / $38) and will happily integrate an LMS such as Canvas.
The book has been written by a number of contributing authors and seems to have been designed mainly as a primer for American undergraduates taking an introductory Psychology module (“Psychology 101”). The general level should be perfectly acceptable for A-level, although you may need to dip in and out of the book depending on the Specification you follow – the 700+ pages cover a wide range of options:
• Introduction to Psychology
• Psychological Research
• Biopsychology
• States of Consciousness
• Sensation and Perception
• Learning
• Thinking and Memory
• Lifespan Development
• Emotion and Motivation
• Personality
• Social Psychology
• Industrial-Organizational Psychology
• Stress, Lifestyle, and Health
• Psychological Disorders
• Therapy and Treatment
The book also comes with a range of Instructor resources (access to PowerPoint Presentations and Test Banks require you to sign-up for a free account), plus a host of free community-created resources.
Other free textbooks you might want to notify subject colleagues about:
• Mathematics (Algebra, Calculus, Statistics)
• Biology, Physics, Chemistry
• Economics
• History