Argos £19.99 'Flight' Bag
This is fine! Meets legal size requirements. Has boards which can be fitted internally with supplied Velcro patches to make it either hard or squashy. One small zipped side pocket, not A4 size, and a big one which closes with velcro and there is also a useful little dog lead clip inside for attaching your car/house keys. Strong black cloth or nylon material, well stitched and should last some time. Good value at the price I'd say.