Thank you for your email.
Your experience in back office will not be a disadvantage to you applying to a front office role. Your experience in the back office should provide you with a insight into Investment Banking and you should be able to draw on your experiences to explain why you want to work in a front office role.
Any work experience is useful when applying for a full-time position and any experience you have will certainly not "tarnish" your application.
We look forward to receiving your application in the Autumn.
Kind regards,