Pecker=Chin (When the English say, "Keep your pecker up", it simply means, "Look on the bright side/keep your chin up"). My Brit mom learned not to use this expression in America, the HARD way! :-))
Knickers=panties ("Don't get your knickers in a twist.")
Rubber=Pencil eraser. We once had a Welsh woman in our department at work, who had no clue what "rubber" could mean here, and would call out, "Does anyone have a rubber I can borrow?", which drew strange looks!
Screw=salary (though I think this isn't being used much nowadays). The woman mentioned above said they used the expression, "He makes a good screw" to mean, "He makes a good salary." No wonder it's not used much now!
(And I always thought SPANISH slang was a minefield!)
Moggy(or is it "moggie"?)=cat
Washing up=the dishes
Washing up liquid=Dish detergent
(The) post=the mail
Dual carriageway=2-lane highway
(on)Holiday=can mean your vacation
WC (for "water closet")=as above
Mum=Mom; though "Ma'am" sounds much like "Mum", too.
Chips=similar to our French fries; often served with battered fish (Fish 'n chips)
Courgettes=(I think) are eggplants
Haricorts vert (actually French)=string beans
Corn=can mean any edible grain; "Maize" is our corn
Detached and semi-detached houses=single-family homes, and the end of a row home
Front garden=really can just be your front yard
Castor sugar=powdered sugar
Front room=living room (used to mean more of a formal one, like our parlor)
Sponge=Can mean a sponge cake, such as Victoria sponge or my fave, jam sponge (actually very good, made in a Crock-pot, believe it or not!)
My Mom, who is not from London, loved to do the Cockney slang thing--"Apples and pears"="Stairs"; "Bee's knees"=cheese. Drives me barmy! ("Nuts")
This is all I can think up off the top of my head...