text: I'm not sure what prevents it from being a department store. I'd say that Garden, Furniture, Beds and the 9 other categories across the top of the web page were all departments. But let's consider alternatives.
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  https://en.wikipedia.org/wiki/Dunelm_Group links to several articles describing it as a "homeware retailer" but OSM doesn't currently document a homeware tag. Perhaps it should.
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  https://wiki.openstreetmap.org/wiki/Tag:shop%3Ddepartment_store suggests we should consider shop=doityourself and gives Homebase as an example. I'd criticise that on the basis that probably 50% of what homebase sells are finished products -- there's nothing you need to "do" yourself to be able to use them,
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  shop=houseware? But OSM's definition is "small items, like cutlery, crockery, cookware and decorative items", Again, I would say that definition excludes a substantial proportion of what Dunelm sells, nor would I search for DIY shops if I wanted to buy many of its products.
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  Off top of head, if I wanted to define something better, I'd probably go for Department Store with a new additional tag "departments=aaa;bbb;ccc,,,," but I'm not aware of anybody having already done that anywhere.