As an employer - Morrison's have a duty of care to make sure that your working envoronment is safe for your health.
If you surgeon/G.P. states that you can return to work, but only on light duties, i.e. sitting on a till etc. then they will have to comply.
If you G.P. put on your return to work note that he recommends you do light duties for say 3 months after your op, I'm sure that your employer would rather have to in work, working than sat at home and getting sick pay.
If I were you I would discuss this with your manager prior to the op and explain your concerns, that way they will have plenty of time to put something in place for you.
I work for a local authority and situations like this, manager will routeinly refer staff to see the occupational therapist to ensure that they are complying with health and safety. I'm sure that they would rather assist you in returning to work than having you sue them for negligence.