Ooh, those make my blood boil and my mates keep emailing me the link saying 'have you seen this?'. Not good for a pregnant woman's blood pressure!
I think if you email them saying that keeping fish in these would be in violation of the 2006 Animal Welfare act and therefore they are not fit for purpose then you might get a response.
Mention Trading Standards, that usually gets their attention. The law states that "The goods must be fit for their purpose. The goods must be fit for all the purposes for which they are normally supplied and, in addition, if you bought the goods for a specific purpose and made that purpose known to the seller then the goods must be suitable for that purpose. For example, if a shop says a particular paint is suitable for outdoor use, then it should be."
I know people that have successfully claimed a full refund after being sold a novelty fish tank so it can and does apply.
Also try sending them links to this British Veterinary Association leaflet and these RSPCA guidelines.
I think the key is to not be fobbed off with 'we have consulted aquatic expert' replies. No aquatic expert would say that keeping fish in these tank is OK, unless it's very temporary.
You're not the only one emailing IWOOT so hopefully they'll pay attention.