In order to get the free 3-year higher education warranty you need to do nothing more than make your purchase via the online higher education apple store with a computer connected to your university's intranet (or by phone specifying your HE student status).
You can buy Applecare in addition for 60 odd quid, with the added benefit of phone support for 3 years and free courier service for repairs (else you can drop off your faulty goods at an Apple Store or authorised dealer for repair should you need it).