I find that Lidl has a good way of deploying staff. It appears that all the staff, including managers (according to the badges) are trained to do more than one job within the store. This might include stock replenishment, removing cardboard waste or being on the till. Each person has an allocated till number and, if the queues build up, a member of staff already on a till will put out a call by number for a particular till to be opened.
There is always someone supervising the self check outs. When I engage in conversation with whoever is serving me, they speak positively about the company, mentioning good holiday allowances, sick pay and pension schemes. Perhaps I'm lucky in my choice of branch (Old Newton Road, Torquay) but it is a model from which other companies could learn, perhaps.