The Police are entitled to ask to see your licence at any time and if you do not have it immediately, you will be ordered to produce it to a Police Station within 7 days. Failing to produce within that period of time is another offence, even if you do actually hold a licence.
RTA 1988 S164:-
(8) In proceedings against any person for the offence of failing to produce a licence it shall be a defence for him to show that—
(a) within seven days after the production of his licence was required he produced it in person at a police station that was specified by him at the time its production was required, or
(b) he produced it in person there as soon as was reasonably practicable, or
(c) it was not reasonably practicable for him to produce it there before the day on which the proceedings were commenced