Children and Young Persons (Protection from Tobacco) Act 1991 http://www.opsi.gov.uk/acts/acts1991/Ukpga_19910023_en_1 states:

Amendment of section 7 of the Children and Young Persons Act 1933
“(1A) It shall be a defence for a person charged with an offence under subsection (1) above to prove that he took all reasonable precautions and exercised all due diligence to avoid the commission of the offence.”

And

Display of warning statements in retail premises & on vending machines

(1) A notice displaying the following statement— “It is illegal to sell tobacco products to anyone under the age of 16”shall be exhibited at every premises at which tobacco is sold by retail, and shall be so exhibited in a prominent position where the statement is readily visible to persons at the point of sale of the tobacco; and where—
(a) any person carries on a business involving the sale of tobacco by retail at any premises, and
(b) no notice is exhibited at those premises in accordance with this subsection,
that person shall be guilty of an offence.

And

Enforcement action by local authorities in England and Wales

(1) It shall be the duty of every local authority to which this section applies—
(a) to consider, at least once in every period of twelve months, the extent to which it is appropriate for them to carry out in their area a programme of enforcement action relating to section 7 of the [1933 c. 12.] Children and Young Persons Act 1933 and sections 3 and 4 above, and
(b) accordingly to carry out in their area any programme which is for the time being considered by them to be appropriate under paragraph (a) above.