In your sentence, I would use "is".
The usage note below is from the American Heritage dictionary: British English does vary from American English on the use of plurals with nouns representing groups of individuals.
Percent can take a singular or plural verb, depending on how the quantity being described is viewed. Very often what determines the form of the verb is the noun nearest to it. Thus one might say Eighty percent of the legislators are going to vote against the bill or Eighty percent of the legislature is set to vote the bill down. In the second sentence the group of legislators is considered as a body, not as individuals. When percent is used without a following prepositional phrase, either a singular or plural verb is acceptable.