OK.
Tailor your CV so that it shows all the things that you do now that are relevant for a buying role.
While you are hunting for a buying job, can you get a part time job in retail? Even if it's only 4 hours a week it may help.
Have you applied to companies in Herts? Mothercare and TK Maxx (I think) both have Head Offices in Watford, and the competition may be a little less there.
There are also some buying roles here
http://www.questsearch.co.uk/
that do not insist on a degree.
In the meantime, do lots and lots of research on John Lewis and prepare yourself for the interview as well as you can. There are lots of sample interview questions on the internet - look them up and prepare answers to as many as you can. Even if you're not asked these exact questions in the interview, it'll get you thinking about what you've done in the past in order to answer the questions you do get asked in the best way possible.
Also, prepare a list of questions to ask the interviewer to show that you are interested in the job.
Lots of luck, let us know how it goes!