American Doctor Who fan here! Had to do some research on what some of those are but do we have access to the following:
crumpets: not really as far as I know
Kettles: Actually yes, a lot of people who live in small apartments or college dorms especially like electric kettles since they don't need to have a full stove to boil water. I wouldn't say their as popular here, but definitely attainable and not all that uncommon.
Cereal that's not sugary: depends on where you shop, but usually yes, however, a lot of times they market it as being extra healthy and all that so they sell it for a higher price, therefore discouraging people to buy it.
Mars bars: not under that name, we have something extremely similar called Milky Ways, but I will say some people report some texture and proportions differences in the two.
Hobnob biscuits: not very well known but technically available in some common American grocery stores at the caveat that they seem to be unnecessarily expensive for cookies, likely due to transportation costs.
Maltesers: Similarly to Mars bar, we have an American equivalent called Whoppers with once again, some difference in texture and a size difference
Wotsits: Yes, under a ton of different names, such as puffed cheetos
Quavers: not really, in theory you can buy them at special places or online but most Americans wouldn't know of them. They seem to have a different shape and texture than American potato chips, and possibly an entire different cooking style/method.
Lucozade: Nope. We have some sports and illness recovery drinks but none are carbonated and they don't have a focus on glucose.
I'm sure for all the "No's" it's possible to get them somewhere with some effort but their not mainstream or easy to access. All of this is based on my personal research and general assumptions about what most Americans know about but I think this is at least pretty accurate.
oh and hot cross buns are pretty easily available but only seasonally, though I'm not sure the popularity of them here. Many of us do know the hot cross buns nursery rhyme, it's one of the ones we're often taught to play on the recorder in third grade and many know the accompanying commonly played in elementary (primary I think you call it) school clapping game.
