I bake a lot and all the seasonings you listed I've easily found or I know I can get.
Onion powder was a bit tricky, but you can sometimes find it in the Indian section (Natco brand). I use
onion granules and it works fine for me.
You can find
Sriracha in the supermarket, but we usually get it from Chinatown (the garlic Sriracha is super yummy).
I make my own applesauce since the applesauce here seems to be sold more to eat with meat dishes. I use my homemade applesauce as an egg replacer when baking.
For the
garlic and
ginger, look in the Indian section for glass jars of the minced stuff.
You can buy pre-sliced packaged onions in you're really sensitive. They have some in the
fresh fruit and veg section (near the packaged salads, I think) and they've got more in the
freezer section.
You can get tortillas here, but don't expect anything authentic. We get
tortilla wraps from the bread section and they work fine for us.
Sometimes I have to hunt for it, but canned pumpkin can be found. It's not popular, it might be seasonal, I can never remember where it is in the shop and the assistants usually have no idea, but chances are, it's somewhere. I've had good luck at Whole Foods.
The main supermarkets are Tesco, Sainsbury's and Waitrose. You can always search online and see if something comes up.
https://www.tesco.com/http://www.sainsburys.co.uk/http://www.waitrose.com/