I didn't find any at my Wilco's, and I didn't have the nerve to brave the crowds at Poundland or the 99p shop :::::shudders:::::
So I went off yesterday on my lunch break to look for those plainy-plain ones at Smiths...
And I found (at WH Smith), with the Christmas gift wrapping stuff, a pack of 3 gift boxes (one a little bigger - like for a sweater or bathrobe maybe, one perfect sized for a shirt, and then the third in the set is smaller like maybe for a scarf or a pair of gloves or something like that) - decorated in a silver foiled holiday design (no extra wrapping required). I think it was £5.59 for the pack, but it was included in their 3-for-2 offer (on a wide variety of holiday giftwrap items) so I picked up two of those gift box packs along with a pack of 6 red foiled bottle bags.
Just like in the US - except pricier, and only choice of one (heee!) festive design. But I'm happy to recycle them & I'm sorted for what I needed!