oh darn it! so i've missed the deadline?
you've missed the UK offer. (it was offered from April 21st through November 11th IIRC)
if i've missed it for October 2015, is it worth purchasing the QSDP?
Absolutely not (well ... depends

)
you can purchase it at your resort.
and the ease of the dining plan was very useful for us, but i'm not sure if it warrants the £800 price tag.
you need to do your maths.
It really depends if you're having dessert when you're on the QSDP or if you end up throwing it away because you're no longer hungry at the end of the meal.
a basic entrée is $10, add $3 for a soft drink (you can get a free cup of ice water at CS restaurants)
so if you don't have a dessert, your daily expenses are around 2 * $13 + a $4 snack + $1 to $1.5 (value for the mug depending on your length of stay)
that's $31.5 if you pay OOP without getting a dessert. (and $25 if you only use your refillable mug at your hotel, and drink water)
if you pay for the convinience of the QSDP you're going to pay over $42 per night, so basically you might waste $10 to $15 per night per person
IF you have dessert, and always go for the most expensive item on the menu (not all CS restaurants have over $10 items) then you might break even, and you can justify the price tag for the convinience of being on the dining plan but you won't save. You'll end up paying a bit more than going OOP.
If you don't, and can settle for an ice water cup from time to time with your meal, and skip dessert, then it's not worth purchasing the QSDP at all.
other option will be to go for a room only reservation on the US website, and IF a deal comes out for october (depends on your dates) then you may call the reservation center and upgrade to a package by adding tickets and the free US dining plan (QSDP for values, DDP for mods and deluxes). In this case, tickets will be US MYW tickets, limited to 10 days max
If the dining deal doesn't come out, or if your dates don't qualify, then just add tickets (US or UK tickets)