i currently work at tesco and i'm 17 almost 18 now....i work on checkouts...
i started when i was 16, i had just started college after GCSEs...i had no previous experience with any companies or employers..
your better off just applying for a proper job there, i only do one saturday a week so it fit's in with college work! and when i move to uni i can transfer my job to another tesco store. I get all benefits that tesco employees receive even though i only work on saturday! which is good! I get £5 an hour currently but it this will increase when i'm 18. All I had to do was fill in an application form with a set of multiple choice questions which you had to pass on, then you attended an interview and they'd ask you a set of questions basically about common sence, what you'd do in a difficult situation and if you work well in a team...
i wouldn't bother with unpaid work experience when you could easily get a paying job there!
what about work experience in a pub? washing up? or in a resturant?...they'll take people with no previous work experience. Also building labourers and farm helpers...
there are plenty of paying jobs out there which will take you on...
+ oh and i applied three times for a supermarket job before i actually got one!....you've just gotta keep trying