Unfortunately, consumer rights are all between the buyer and the supplier (not the manufacturer)
The quoted link only refers to distance sales (internet / phone orders) and the returns info only relates to returning orders to the original supplier if immediately found to be faulty (or you are entitled to change your mind and return for a refund)
The sale of goods act relates to all purchases and covers purchases for up to 6 years but again only covers returning goods to the original supplier, not the manuafacturer / agent.
http://www.which.co.uk/consumer-rights/regulation/sale-of-goods-act/
The seller is the one who is responsible for refunding, repair or replacement and although the buyer can request a preferred remedy, the seller can choose the cheapest for them. If you return a loco they can choose to refund and make a deduction based on the use and age (so you could end up with a partial refund and without a loco which is virtually impossible to find and replace)
Returning to DCC supplies is covered by the Dapol warranty which is addition to your statutory rights, therefore they are free to impose their own terms