Check out the following online specialist foodie shops the supply American treats in the UK:
"American Sweets": www.Americansweets.co.uk
(Just ordered "Duncan Hines White Cake" mix from them, should be here on Tuesday! How's that for a quick result?)
"Let's Eat American": www.letseatdirect.com
And, mega thanks to Nanny Debbie who spotted "Betty Crocker White Cake" at Bakers & Larners!