If you had brought them into my shop with the sole coming off the first thing I would have done would be to offer you a repair, if you had accepted that then the repair would have been carried out free of charge and your shoes would have likely been fine. If you had wanted an exchange then you could have asked for one and you would still have been offered a repair but if you had insisted on an exchange you would have got one. if you had brought them in and asked for a refund and continued to ask for a refund at ever offer of a repair or exchange then you would have been given a refund as ling as I thought that the sole coming away had been a genuine fault.
It does sound as though the assistant/manager, offered you a repair based on their insepction of the shoe and the decision that perhaps you had tripped and caught it and caused the damage accidentally, or perhaps you had driven in them and caught the sole on the car pedals.
In any case, with business how it is all retail stores will be doing everything they can to keep refunds down to a minmum.
the 28 day returns policy on goods fit for resale applies to items you but that you do not use, keep the recipt for and do not remove the labels on, this is not a right in law it is simply a custoner service policy, which of course varies from stoe to store, some allow 14 days some 28. You cannot demand that anything is returned under that policy if you deem it to be faulty or fit for purpose.
How that you have accepted the repair i am not 100% sure that you can change your mind as noone has ever changed their mind after agreeing to the repair, i know it is a section in the sale of goods act but neve3r had to refer to it.