You can find the Exterior angle in a regular polygon by using Ratio Grids if you want to.
Do you know your Circle Theorems? (Printable Poster)
I am grateful to Mr. Holding who teaches locally for this link. So simple! Lots of GCSE questions presented one at a time. A couple of examples are reproduced below.
The Proof Questions on this site are invaluable – the best collection I have seen.
Parallel lines activity
Mathwarehouse.com is *very* buggy website but oday one of my students did the parallel line problems and thoroughly enjoyed them.