Pop Tarts yes but generally just the plain brands ( No S'mores unfortunately
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)
Twix yes but not the many varying incarnations of Twix you tend to get in the states, I've only seen the plain old caramel.
Mountain Dew, no such luck I'm afraid. I've had an equally difficult time getting Root Beer. I'm told Sainsbury's sell it, but there are none within easy distance of my home so I suffer without
Ranch Dressing, yes Tesco sell Newman's Own but I personally hate this brand. It certainly doesn't taste anything like Hidden Valley if that is the kind of ranch taste you prefer.
Goldfish Crackers, no. But they have quite a few different types of cheese flavoured crackers.
Peanut Butter, yes, but I've not seen any Jif. You can get Skippy (but again, not the multitude of kinds... crunchy, extra crunchy, extra smooth, low fat...just plain old Skippy) or the English brand SunPat (eugh!).