No, it operates on trust and on tagging of some items that are most commonly stolen. For most items, there's no security apart from one employee who might or might not be watching at any given time. That employee is there to disable tags, check customers for age-restricted items and deal with cases in which the self-service system doesn't work properly. The exit to the self scan area will have a scanner that sounds a minor alarm if someone passes through it with a tagged item, but that's it. No barrier.
Oddly, the system works well enough. Some of the tagged items are a bit surprising. I bought some underpants in Tesco a little while ago and they were tagged as items at high risk of theft so I had to have the Tesco employee disable the tag for me. They were Tesco own brand underpants. £5 for a pack of 3, I think. Something like that.