The large supermarkets in SD (Nacional, Bravo, Carrefour) might have marmalade (of sorts) and English tea - brands like Twinings - and possibly spam as it is an American thing (for want of a better word).
Specialist food places in the capital where you might find some of the things on your list:
L'Epiciere de l'Orient - Plaza Paseo de la Churchill on Ave Churchill esq. Pastoriza, next to Yogen Fruz. It is mainly a Middle Eastern deli but they have stocked English teas and other exotica, although it's a good few months since I last visited.
Omaha Steaks in the same complex might have the mustard powder.
EAT on Bolivar/Betancourt just after you cross Lincoln. I saw they had Lemon Curd there (and believe it or not, resisted the temptation) and they probably have marmalade too.
Marmite is still an informal import, depending on the kindness of friends who come over from Canada/US/UK and dependent territories.
Cookers with built in toasters are available, though as Grahame says the toaster bit tends to be in the bottom drawer. Mine has one, and I've never used it, I still use the stovetop camping doodad.
I think custard is not too likely - you could try making it from scratch!
And as for Guinness, there have been threads about this on DR1 in the past and the verdict was that it did appear in SD supermarket beer aisles at erratic intervals. Depending on your level of desperation, this may be of interest: "Guinness" is sold in Haiti but whether it bears any resemblance to the real thing is another matter. Maybe you would like to take a trip to the border for purposes of research?
