My daughter was asked to leave a pet shop as she was unaccompanied by an adult and was under 14. She was in there with the intention of making a genuine purchase...
It may be because the shopkeeper didn't want to commit a criminal offence under s.11(1) of the Animal Welfare Act 2006:
A person commits an offence if he sells an animal to a person whom he has reasonable cause to believe to be under the age of 16 years.
ETA in response to comments...
It's not that uncommon for some shops to impose a blanket ban or a "maximum number at any one time" policy for unaccompanied children when, for example, they have been subjected to antisocial behaviour or lost significant amounts of stock through shoplifting. I cannot find any law that make this type of policy unlawful.
Also, in the UK, when shops "exhibit goods for sale" it's an invitation to treat and they are not obliged to sell anything to anyone - unless it discriminates against a protected characteristic (which in this scenario does not as appear to be the case according the OP's answer).