Having looked into this matter I understand that when the Animal Welfare Act (2006) was being considered, there was a proposal to ban giving away live animals as prizes. The government of the day consulted on this proposal and eventually decided against an outright ban, instead making it illegal to give such prizes to unaccompanied children under the age of 16. There are no current plans to revisit this legislation.
Fairgrounds are, however, licensed by local authorities, which have the power to impose conditions before allowing the event to go ahead. This could include a prohibition on giving away live animals as prizes. If you are concerned about this practice happening locally, therefore, you may wish to raise the matter with the council.
Goldfish, like all other vertebrate animals, are protected under the Animal Welfare Act. This means it is an offence to cause them unnecessary suffering or fail to provide for their needs when responsible for doing so. Breaches of this law can be investigated and prosecuted, so I would encourage anyone who comes to own a goldfish to ensure they look after it properly. The RSPCA makes guidance available on its website about how to care for freshwater fish, so this could be a helpful place to start.