I think diners are often interested in regional specialties, so you could present it that way (even if the dishes are now national).
Yorkshire pudding
Lancashire hotpot
Cornish pasties
Haggis
Pie & mash & liquor
Bakewell tart
Eccles cake
Eton Mess
Pork pie
Scotch broth, mulligatawny etc
I think the Italians are a bit more enthusiastic about offal, so also
Liver & onion
Steak & kidney pie (with mash and liquor?)
Haggis!
Black pudding (we see a lot of mortella on menus here, so let's return the favour)
White pudding
And we haven't even got to the a classic, well-made British sausage - truly, a thing of joy.
Pea & ham soup
suet pudding (sweet and savoury)
sponge puddings
FISH & CHIPS!
Scampi
Plus championing British produce to replace things they already have;
Welsh lamb
Scottish beef
British cheeses
British beer & cider, gin and whisky
There's a wealth of things to work with here. For kor inspiration have a peek at the recipes books from the top chefs offering 'modern British' cuisine.
And you can always horrify them with a British-style spag bol or macaroni cheese!
