Voting-wise as the conditions seemed to rule out UK fast food (we don't have many drive-thrus - certainly no chain can be relied on to have them anywhere near you), I went with Dairy Queen, mostly because I have good memories of the times I went there in US, but also because I love that there's a first-food place where you can get like, a full-on birthday cake or whatever (the ones I went to were mostly in Indiana, and were all decently-sized and had burgers and drive-thrus, interested to read that isn't always true on the East Coast).
(Also all the best UK fast food isn't named chains, it's local places which chain-like in that they will have a menu 90% similar to other shops in the same "genre" of food - i.e. Turkish kebab, Lebanese kebab, Fish & Chips, Chinese (actually Anglo-Chinese - very different to US Chinese - a lot healthier, a lot less sugar, many just totally different dishes), Indian (actually Anglo-Indian/Pakistani/Bengali/Sri Lankan), and so on. I think that's true to a lesser extent in the US, like local pizza places will always be better than pizza chains, but it feels like in the US chains are better-quality, relatively - the only UK chain which really competes with local restaurants well is KFC and that's solely frying chicken is more of a science than an art.)