Firstly, have not used the dining plan, but this is what i have learnt. Disney will not let you have the dining plan through them unless you book a package from them. This means the room and tickets directly from them, the flight you would book seperately yourself. If you are booking a package with a uk company, then you have to book the dining plan with that company.
I have booked a package with virgin and although not in the brochure I asked about the dining plan. They included it but when I did the maths, it was far more money than disney were charging per day, or First Choice or anybody actually! I rang to get it taken off my booking and was charged £25 to do so, even though they are supposed to price match with other companies. this does not apply to seperate components apparently so i was stuck.
When i worked out what my food costs would be, i think it is going to work out cheaper to pay as I go anyway, because there is no way i can eat 3 courses at a sit-down meal. I also did an estimate with the help of allearsnet and their menu pages and I am happy i will be better off.
I think if you want to eat expensive meals and a large quantity, then the dining plan could well benefit you so it might be worth working out the maths first before getting charged like I did
Hope this helps,
Claire xx