A few dainty sandwiches (maybe 3-4 different “rounds” of bread, with 2-3 different fillings, 1 per round, cut into 4 triangles each or even into smaller fingers). Fillings like plain ham and (grainy) mustard, maybe just ham for those who don’t like the mustard (could alternate those as 1 white and 1 brown bread round to distinguish), egg mayo, tuna mayo, tomato (very last minute made) with lots of freshly grated pepper, smoked salmon and cream cheese, thinly sliced cucumber etc. Some of these could be open sandwiches on nice sourdough or brown bread rather than sandwich bread, if you have it.
Some dainty sized scones with real butter, clotted or whipped cream and jam for people to fill themselves.
And some kind of dainty cake things - meringue nests with cream and strawberries, chocolate brownies, lemon drizzle squares, those pastry/raspberry jam/almond mix buns in small squares, individual cupcakes, individual cheesecakes, individual fruit tartlets or small slices of cakes, maybe a few biscuits or dainty cookies etc....
And in terms of quantity, I would allow maybe 1.5 “rounds” of sandwiches (each round being 2 slices of sliced pan and it’s filling) per person, 1-2 scones and 2-4 dainty cakes/pastries per person. If you know your guests well, you can judge if they are more likely to veer towards savory or sweet and adjust accordingly.