According the press reports this sandwich company pays at least the minimum wage. And as it's such a big company, I think that's probably true - it would be hard for them to get away with paying less.
The point about in-work benefits such as Working Tax Credit is (as I have mentioned before) that it means that no-one should even find themselves better off on benefits than they would be in work. Of course, human nature being what it is, that still doesn't guarantee that there won't be some lazy people who would rather stay on benefits and have less money!
But on the whole, in-work benefits have been effective, and both Tory and Labour governments have recognised that over the past 30 years. And as a result the UK unemployment rate has fallen from a peak of 12% 30 years ago, just before Family Credit was introduced, to c.6% today.