I think AA are interpreting this incorrectly. The rules for connecting flights and ADP exemption state:

In addition to the time related criteria, the agreement for carriage must be evidenced by a ticket which must show the:

airport from which the passenger intends to depart
date and time of his intended departure, and
airport at which he intends to arrive
The connected flights must be detailed on the same ticket or conjunction tickets to qualify for the exemption. Tickets can only be regarded as conjunction tickets if:

a. they are in one booklet, or

b. where they are in separate booklets:

each refers to the other and states that they are to be read in conjunction, or
there is a summary of the flights constituting the passengers journey including the flights in question.
c. where the tickets are purchased online, they are purchased at the same time through the same portal.


(From http://customs.hmrc.gov.uk/channelsP...505#P280_14691 )