How does one bring this 'illegal' behavour into the public domain ?
You seem to have answered that half of the question.
I would back up your telephone call with a written confirmation that you have rejected the goods under the distance selling regulations.
The retailer has the opportunity to exchange the goods at extra cost for the larger model or to refund the full purchase price within 30 days.
Whilst you may get some verbal bluff over the telephone, it's unlikely that a reputable retailer will knowingly break the law and I wouldn't expect it to go to court. But you may have to demonstrate that you mean business.