I'm sorry I don't have a firm answer to your problem, but the dining plan is sold as part of a package, so I dare say Disney would be correct. When we purchased the dining plan for our upcoming trip, we purchased it directly from Disney. But we purchased our whole vacation package directly through Disney as well. In order to get dining, you have to stay at the resorts--it's tied together, so Disney isn't going to directly sell you dining plan without directly selling the resort stay to go with it.
If you're buying your trip through a travel company, you should be able to purchase the whole package (room/dining plan/tickets). If your travel company is claiming they can't sell you part of a package (i.e. the dining plan), I'd start looking for a different travel company. I obviously don't know for sure, but being in the UK shouldn't make a difference when booking the dining plan.
Someone with more knowledge feel free to step in here!
P.S. You might get more responses if you posted this as its own thread.