There is no law stating precise sizes and types of cages for hamsters. The animals welfare act states :
9Duty of person responsible for animal to ensure welfare
(1)A person commits an offence if he does not take such steps as are reasonable in all the circumstances to ensure that the needs of an animal for which he is responsible are met to the extent required by good practice.
(2)For the purposes of this Act, an animal's needs shall be taken to include—
(a)its need for a suitable environment,
(b)its need for a suitable diet,
(c)its need to be able to exhibit normal behaviour patterns,
(d)any need it has to be housed with, or apart from, other animals, and
(e)its need to be protected from pain, suffering, injury and disease.
https://www.legislation.gov.uk/ukpga/2006/45/section/9
You would need to prove that the cages were not suitable environments and that the animals could not exhibit normal behavior patterns if kept in these cages. A lot of these things also depend on the way the hamster is kept when not in the cage. Some animals will have a lot of out time / playpen time and just sleep in the cage. Every individual differs in their needs and each one needs to be housed in a cage appropriate to its needs which can change over time.
You will have a difficult task ahead of you. This is only one store selling what is provided to it by manufacturers. You need to tackle the root source - the cage manufacturers and not the stores who will provide cages that are affordable to all types of customer. Education is the key to this in my opinion so that the customer will recognise when a cage is unsuitable and will not buy it so the companies will stop stocking them.
Research & stats on hamster welfare needed
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