If you haven’t notified a garage that you wish to keep old parts from your car once they have been removed, the business should keep the parts until you have collected the vehicle and will normally dispose of them in a reasonable timeframe after the repair for space and safety reasons.
If the removed components are no longer available to view or keep, contact the business to find out if there is any other evidence they have kept showing their condition (e.g. videos or photos). If there isn’t any such evidence, you may wish to consider discussing with the business, before raising a dispute with The Motor Ombudsman, as to whether they may be open to some form of remedy to recognise the fact they have been unable to provide the old component(s).
Read The Motor Ombudsman’s article “Looking the part” for further information.