A dress by Beaulah London called "Aahana", seems to have been bought by multiple royal ladies, some wearing the short-sleeved version, some wearing the long-sleeved style. Here's the dress:
And here are the royal ladies wearing it:
From left to right, Princess Mary of Denmark, the Princess of Wales (Kate Middleton), Princess Beatrice, and Lady Frederick Windsor (Sophie Winkleman).
You can see why they buy it: it's well-made, formal (calf length), and dignified enough for any day time occasion. Kate wore it for an official visit to a school, Beatrice and Sophie wore it to Ascot. It's still available on the Beaulah website - it will set you back £720.