The info you need is here
Embassy of the United States London, UK - Visa Faqs
However I'm curious as to why you think you are going to have a problem?
The FAQS state that if you traveleld by commercial carrier then its not a problem even if you didn't surrender your I94W. I know this for a fact , my OH travels to the states on business all the time , he always keeps hold of his I94W in case he travels again within its validity. If not he simply bins it.
Being doing this for years.
The jury is still out as to whether the address in Kentucky is actually one large garbage pile or if they actually do anything with returned cards
