Rather than cancel and forfeit the deposit, Isn't it UK Consumer Law that if you take delivery of the item as an online purchase, you have the right to reject it if it doesn't meet expectations as to quality, performance, fitness for purpose?
You might have to swallow the postage costs which is fair, but you would be lawfully entitled to receive a refund in full from the supplier
The Consumer Contracts Regulations
When you buy goods online you have additional rights to return them.
This is because your decision may be based on a brief description or a photograph – so what you receive isn't always quite what you’d expected.
Under the Consumer Contracts Regulations, you are allowed to return an item if you simply change your mind.