davidg_richmond has given you excellent advice, and I suggest that you pay particular attention to what he said with regard to cancelling a finance agreement. Many people fall into the trap of thinking that such an agreement is with the retailer, and that when they are in dispute with said retailer defaulting on the agreement is OK - which is very far from the truth.
A finance agreement is made with a finance company, and it's to that company that you have a legal obligation. The finance company is a separate entity, and a dispute with the retailer does not concern them.
If you don't like the offer that Dixons make you do as davidg_richmond suggests, and explain to them why you think you should get more. You may well be offerd a camera that has a higher specification than the original, but you are not entitled to expect it. The matter of compensation for the many weeks that you have been unable to 'enjoy' (to use the legal word) the camera is a separate - although related -issue, and one which you might have to pursue in court - Dixons are not legally obligated to compensate you under the terms of the Sale of Goods Act, although as Gandalf says, they might feel that in the circumstances some form of recompense is warranted.