This slim, red Collins Rubaiyat has illustrations from Marjorie Anderson and an introduction from Laurence Housman. It is undated. It seems to be from their ‘Collins Greetings Books’ series.
This edition was donated by Ian Chadwick. Thank you!
It doesn’t use Roman numerals for numbering, which threw me for a moment.
It seems to have originally come with a case, though my copy does not have one. There’s a few Abebooks listings for seemingly this same edition that comes with a patterned case with a cut-out for the title.