Yeah, it's like people who spend time around campfires and have watched American media are all going "let's make s'mores" [1], and then they realize that "graham crackers" [2] are a mystery ingredient that nobody knows anything about.
Digestives [3] are the typical substitution in my experience, but again nobody knows how close they're getting. They look thicker, to me ...
Well also that you can buy graham cracker crumbs, for making things like pie crusts. My friend gave me a weird look when we went shopping and I picked up whole crackers. And then the revelation that graham refers to a type of flour and is not in itself a brand. And Kelloggs sell the crumbs? Wild.
A biscuit base in the UK would usually require a pack of digestives and a rolling pin. I suppose some supermarket sells crumbled biscuits but...
As an aside, Golden Grahams used to be popular in the UK and I don't think anyone stopped to ask what the name meant.
Digestives are a bit thicker, but the ones I had while over there weren't substantially so. You're less likely to get the shared experience of dealing with the goopy mess all over your fingers because your graham is shattering at the first bite.
Digestives actually is a pretty good sub! Yeah theyre thicker but I think the most substantial difference is that graham crackers are significantly harder and less crumbly.
I think another option is chocolate 'malted milk' (in the UK) - depending on your preference for ratio of biscuit to chocolate to marshmallow. Leibniz will have more/thicker chocolate, but malted milk will break a bit easier in the mouth (softer/crumblier biscuit).
Digestives [3] are the typical substitution in my experience, but again nobody knows how close they're getting. They look thicker, to me ...
[1]: https://en.wikipedia.org/wiki/S%27more
[2]: https://en.wikipedia.org/wiki/Graham_cracker
[3]: https://en.wikipedia.org/wiki/Digestive_biscuit