Your question, as it stands, is too vague to provide a definitive answer to.
A person who simply asks to see your 'proof of age', and who has no wider role than actually supplying you with age restricted goods (or with an age-restricted service), such as a shop assistant or tattooist, isn't obliged to show your their own ID.
However if a store's security guard interacts with you in any way (such as when supporting a shop assistant who is seeking proof of age from you), you can demand to see his SIA badge.
Similarly, if you were to be stopped by a police officer who was concerned that you were wrongly in possession of age-restricted items, you could demand to see his warrant card.