From AQA comes an outstanding resource, GCSE Mathematics: 90 maths problem solving questions.
The new GCSE specification includes: “relate ratios to fractions and to linear functions”. Use the excellent Desmos graphing calculator to demonstrate examples; taking an exemplification question from AQA I created a Desmos page to illustrate the problem.
Just as one can always find something on Nrich, the same is true of CIMT. From the Centre for Innovation in Mathematics Teaching we have Ratio and Proportion from the Year 8 book (UK age 12-13) and Variation from the GCSE course (UK age 14-16). As well as the Year 8 text, further resources are available such as Activities. Other teacher resources include slides and Revision Tests (you will need the CIMT password for some resources).
These sorts of problems must be on everybody’s Mathematics specifications all over the world! In the UK there is an increased emphasis on Ratio and Proportion in the new GCSE specifications; a link to these resources has been included on the GCSE New Content page.