This is actually something that fascinates me and I'd like your own opinions on the subject. I list here things I feel are quintessentially English (with apologies to the Scots, Welsh and Irish as they have their own uniqueness).
First of all, it's got to be a cup of tea (yes, I know it wasn't originally made here, but it has become an integral part of 'Englishness').
CUP OF TEA
CRICKET
STRAWBERRIES AND FRESH CREAM
RED BUSES
BLACK TAXIS
PUBLIC SCHOOL
STIFF UPPER LIP
LOVE OF CATS AND DOGS (though this has been questioned these past few years due to the amount of cruelty cases, but generally it still holds)
TV DRAMAS such as All Creatures Great and Small
MARMALADE ON TOAST
ROAST BEEF AND YORKSHIRE PUDDING
FISH AND CHIPS
MUSHY PEAS
SALT AND VINEGAR
PORK PIES
MEAT & POTATO PIE
HOTPOT
RICE PUDDING
BREAD AND BUTTER PUDDING
TRADITIONAL ENGLISH BREAKFAST: Tomatoes, Fried egg, Fried or grilled Sausages, Bacon, Mushrooms, Toast.
ROSES
THE ROBIN (England's national bird)
TROOPING OF THE COLOUR
BBC
Feel free to add as there'll be lots I've missed.
Ian