I recently (yesterday) had a similar issue with an order from Jessops
I thought the 6 months interest free credit after paying a deposit by credit/debit card was a good deal.
Nowhere did it clearly mention that if I was accepted for the credit option it would delay my order until I'd signed and returned a separate credit agreement (until I receiver the numerous confirmation e-mails).
I contacted Jessops using their "contact us" option on the website and asked to cancel the order, explaining why (due to the additional delay).
Jessops have cancelled the order (as they are required to do under the distance selling regulations) and I reordered paying in full by credit card.
The order is now on it's way, weather allowing
If you don't sign and return the finance agreement then the transaction is void in any case as it forms part of the purchase contract.
If you can afford the camera and still get the £75 discount you should be able to reorder it as a separate transaction.
Under distance selling regulations if you cancel an order before the product is shipped you should not be charged, if the product is shipped then you are liable for carriage costs but as long as the product is unopened you shouldn't be billed for anything else if you return it.
Personally I've found Dell are fine as long as you either pay in full when you purchase a product or are a registered as a business customer.