Based on previous years, there are two ways in which you can get the free dining.
1) - Around mid August, the UK travel agents will offer free dining, providing that you book by a given date, you stay between certain dates (usually mid August until the end of Sept), and you do not have a split stay holiday. You must also purchase Disney tickets for the full duration of your holiday. Last year it was only available for a moderate or better.
2) Around May/June time, may be earlier, WDW will offer free dining to guests staying between certain dates. For this year, I think it was available for all types of property. Someone will correct me if I am wrong. So the chances are that if the UK TA's offer it this summer for next summer, then Disney will offer it late spring next year for the same period. Peeps who book DIY holidays, usually book a hotel room anyway, then when the offer comes along, they cancel and re-book the room with Disney, so they get the free dining.
Hope this helps. I've never booked with Disney myself, so I'm only going on what I've read.
Most of us on this thread re hoping and waiting to see if free dining is offered by UK TA's. I havemy Tesco points converted and I am waiting to see. The only thing is we fly on 22/08 and I have a feeling they will go on sale then. Due to state of the economy both here and in the US, I think bookings will be down and Disney will be wanting to tempt us.
Last year, with VH there was also get three nights free when staying at Port Orleans/Riverside too, and both offers could be combined.