A few weeks after England Athletics receive your affiliation fee from your First Claim Club, England Athletics will send you an information pack which includes a card known as your Competition Licence.  This card has your Universal Runners Number (URN) on it.  In my experience it is the number that you need when entering races and not the card.  The number is increasingly being used by race organisers to clamp down on abuse of the discount on race entry offered to affiliated runners. Keep the card safe.