There is no harm in applying just to experience the selections ... the early phases are usually fairly in-expensive but not cheap either (£50 - £80).
By all means try applying with the intention to win in your last year, make sure if you can that phases will not occur shortly before/during exams ... the uni course (or equiv.) is far far more important.
My uni openly encouraged applying for experience, they certainly won't be unhappy if you wish to leave if you got a place but they'd advise against it. A qualification like that could prove useful if you did fail a medical or something in the future - as Sagey says...
SS