From Andy Lutwyche on TES, his ‘Show that…’ resources are designed to encourage students to explain their working. For each resource example solutions are given. The series several resources including Fractions, Forming and Solving Equations, Surds, Equations of Lines and Circle Theorems.
Also on TES resources, a single worksheet of 7 varied show that GCSE questions.
For Algebraic proof, try David Morse’s Maths for Everyone GCSE questions by topic; use the filters Higher and Algebra and scroll down to Algebraic Proof.
From Dr Frost Maths see Full Coverage: Algebraic Proofs a compilation of GCSE questions including answers.
On Dr Austin Maths, try her Algebraic Proof Practice Strips.
Or use Algebraic Proof, Topic 1 for GCSE higher on Jonathan Hall’s Mathsbot