I am not from the UK, but was brought up reading pony books and Famous Five and Swallows and Amazons.
Now we are in spring I am becoming inspired by te thought of a proper picnic.
What is in yours?
In mine (literature inspired) I would have;
French onion soup laced with sherry in a flask (Thank you Rosamunde Pilcher 'Winter in Solstice')
Proper ham rolls with fresh bread and english mustard
Boiled eggs with a screw of salt
Ripe plums or apples or pears
Real gingerbread
icecreams
Strawberries with cream
Orange ade