this page has all the info you could possibly need to know about aea's : they are supposed to be accessable to any syllabus, they are designed to challenge the top ten percent and this one tells all about steps but if you want more info i would email the cambridge students union - the maths undergrads will have done the test and will be happy to answer questions
kings, UCL, imperial and many other unis say that if u want to study maths u have to have further maths or aea or step. is that all still true if u are doing a physics a-level also are aea just a normal test that is done at ur college