Firstly: what type of bread? Must it be white, brown, wholemeal or granary? Are there any significant differences between bread in the US and UK?
In general, the type of bread you would use for a sandwich where the contents of the sandwich are supposed to be the dominant flavor.
The ‘ideal’ bread is soft (baguettes are too crispy for example), moderately dry, moderately dense (you want the air bubbles to mostly be small and relatively uniform, so not something like ciabatta), and mildly flavored.
The ‘classic’ PB&J would have been made with cheap white bread.
Pre-sliced multigrain wheat will will often work well too. Rye or pumpernickel may work, but ire a bit stronger flavored than most Americans would expect. Good pita bread (sometimes called ‘Greek bread’ in the UK I believe), while not traditional, also works well in my experience.
Secondly: Butter, margarine, nothing. Should I butter my bread before adding other spreads? (Idiomatic combinations of peanut butter, jelly, and bread suggests nothing) I guess it is peanut butter on one slice and jelly on the other.
No butter, margarine, or any other such toppings. We simply don’t really butter bread for sandwiches here in the US (mayonnaise is the norm if we need to soften the bread a bit), but even if we did, a PB&J already has enough oil and water in the ingredients that the bread should not need to be buttered. You also generally should not toast the bread either (if it’s too dry it won’t do well with the moisture from the jelly).
Thirdly: Crunchy or smooth peanut butter. These are the two types that are generally available in the UK - is it the same in the US? Is one type preferred, or is it a matter of taste?
Primarily a matter of taste. You can, in theory, even substitute other nut butters for peanut butter, though based on my own experimentation the only alternative to peanut butter that works well in terms of taste is cashew butter (all the other nut butters I’ve tried were either too bland, or ended up too sweet).
Fourthly: Jelly. I'm aware that this is the jam-like spread (and not the gelatine-based substance) but what flavour? In my local supermarket I can find "seedless raspberry jam" and (in a section for sauces to serve with meat) "redcurrant jelly". Are either of these acceptable approximations of the concord grape jelly that I understand is traditional? How much jelly should I use: a thin scrape or layered on thickly?
‘Jelly’ is jam, but with the fruit pulp filtered out (so it’s less ‘chunky’). That ‘seedless’ raspberry jam is probably actually jelly, not jam.
As far as the actual choice of flavor for the jelly, it should be:
- Primarily sweet, and slightly tart, but not savory (the peanut butter is supposed to contribute the savory aspect).
- Relatively ‘bold’ in terms of flavor. Delicate flavors just don’t work as well here.
- Something you actually like.
You’re correct that grape jelly is the traditional option (though the fact that it’s concord grape jelly has less to do with taste and more to do with historical availability of grape cultivars in the US), but all kinds of other flavors work. Blueberry jelly is relatively popular around where I live. When I was a kid I often had raspberry or blackberry jelly instead, and always liked the blackberry better than the raspberry because it had a stronger flavor. These days I prefer lingonberry jam or Key lime marmalade (the first because I just love lingonberry jam, and the second because I love citrus but orange marmalade is not quite strong enough for my tastes in terms of flavor). One of my friends swears by cranberry jelly, and a couple of my friends love using sour cherry jam. I even know people who use apple butter or fig butter instead of jelly.
The only types of jelly I’ve tried on a PB&J that I would consider truly not suited for it are mint jelly and hagebuttermark (a Swabian jelly made from rose hips), and both were cases where the flavors just didn’t blend all that well with the peanut butter flavor. I would also expect tomato and pepper jellies to not work very well either for the same reasons.
Finally: Am I overthinking this?
Probably, but overthinking things on occasion is not always a bad thing.