That's a tough one! Not knowing exactly what is available in the UK, I may mention some things that are not unique. Also, I want to preface this by saying...not all the food I listed is what I would consider good food. Rather, they are iconic meals/snacks. At some point in my life, I thought all of them were good, but that changed as I got older. So, moving on...
As far as microwave meals...it's hard to get less iconic american than Hungry Man Fried Chicken or Salesbury Steak meals. Mind you, they are NOT GOOD...but they've been sold in the US since the 50s (where originally they were heat and eat in a conventional oven only).
Of course, there's Popcorn. Orville Reddenbocker would be the iconic American brand to try, but I prefer Act II Movie Theater Butter myself.
Another idea. Grab a bag of restaurant style white corn Tostidos, shredded chedder or mexican blend cheese, and bottled jalepenos (mild or hot, your choice). Spread the tostidos on a plate, then cover with cheese and place a jalepeno on each chip. Microwave for 30 - 45 seconds until cheese is bubbling. Quick and dirty nachos. Dunno how "american" that is...but it's something I enjoy making!
Stouffers is another iconic American brand. Their meatloaf and mashed potatoes is very good!
Bagel Bites and Totinos Pizza Rolls. Food I grew up munching on with friends on summer break days as we played video games together.
For soft drinks, try Mountain Dew and/or Dr. Pepper. Not sure if either is available in the UK, and they both are rather popular and...unique...tastes.
Chef Boyardee (will be in the canned foods aisle). I'd recommend the ravioli.
Hot Pockets. I'd recommend the pepperoni and sausage with cheese.
White Castle cheeseburgers. Though, they aren't the best. In fact, I don't think they are good at all! But, every few years the picture on the box will sucker me into buying some!
If you really want to try steamed mini burgers (which is what the White Castle burgers are), find a Krystal. They have a unique menu.
http://krystal.com/menu/
There are 3 in Orlando. Also there are 3 in Clearwater.
http://krystal.com/locations/
Anyhow, that's all I can think of at the moment, hope it helps!