That sounds wrong, its only £12 per night difference between the quick service dining plan (two QS meals 1 snack) and the regular dining plan (1 QS, 1 table service, 1 snack). So even for four adults it would only be £672 to upgrade.
I think you were looking at the deluxe dining plan upgrade, which is £70 per night (so £43 more than the quick service), the deluxe plan is three table service meals including starters (appetizers) per day. So upgrading to the deluxe plan for four adults would be £2400.
I took the prices from here
http://www.thedibb.co.uk/forums/Disney-Dining-Plan-p-125.html as I couldn't find the UK prices on a Disney site.
You might want to look again at getting the regular plan as it can be quite nice to have a sit down meal or a character buffet a few times on a trip.
Also remember that you can now use the dessert credit at a quick service meal for any snack. So you could have French onion soup at Be Our Guest in place of dessert or onion rings at Flame Tree BBQ.
Swapping a QS credit for three snacks isn't great value if you were paying for the DDP, but compared to wasting a credit if you have it for free, its better than not using it at all.