If you are going for dinner, try to get an ADR for about 1 1/2 hours before Illuminations. You can go outside right before the show starts and have an amazing view from the observation deck.
I recommend the Fish and Chips, but it's better and cheaper at the take-out window. Either Bass ale or Guinness are also very good complements to the meal.
I was disappointed in the Fish & Chips..they were way too greasy. I noticed the bangers and mash at the next table and vowed I'd order it if we go back. DH was also unimpressed, so I'm not sure I'll be able to get him back again...