The process is simple enough. You, the customer, scan the barcodes on all of your shopping and the machine calculates the bill it will charge you. This is not self service as the machine calculates the bill. The customer would have been carrying the goods in any case so, the service is to Tesco not the customer.
This service to Tesco involves the correct use of a machine for which the public have never been correctly trained to any certified level of competence. This is a serious breach of the principles of Health and Safety Legislation as the machine apparently contains a laser for scanning the barcodes. Customers are being expected to operate potentially hazardous machinery without training. Tesco does not extend workplace insurance to such customers and so the entire process becomes unpaid labour carried out at your own risk. Since the isles were blocked for a stock take - placing hazards in the path of the public - it seems evident that Health and Safety is not a principal concern at Tesco.
The machine calculates incorrect bills placing items on the printout that were not purchased or scanned through. The automated process charges what the machine determines should be charged. When the machine calculated charge is incorrect this requires a visit to customer services in order to get a refund. Yet, customer services are not available for all of the hours that the machines are in operation. Thus leading to a significant cost being placed onto the customer in being made to pursue trivial billing problems at personal expense. This shift from being a customer to being an unpaid employee is an obvious way for Tesco to reduce staffing overhead but also requires a public subsidy to operate.
If there are no permanent staff around then a bill can not be disputed. This, in principle, would seem to lead, logically, to the situation where Tesco will claim that the machine was correct and the unpaid staff or customer wrong. Should someone be paying by credit card, they may never know that their till roll was incorrect.
While it may be inconvenient to Tesco, the most sensible route for anybody using such machines would be to check the till roll before parting with any cash or credit card details. Currently this is not possible - yet it requires only a small system change to print the till roll out for checking before taking any cash. Financial mistakes might well be trivial on an individual basis but will accumulate to millions over the course of a year and many locations. This will make profits healthier but is a serious and systematic abuse of consumer rights.