Hi,

The dress code ‘lounge suit’ for women is distinct ‘cocktail dress’ and 'black tie'. Your dress should preferably be knee-length, and you should try and avoid the sequins, very similar to your business dress! If the reception is at an embassy then there is likely to be an inherent atmosphere of formality, so don’t go too wild with the colours – the men around you, if not in national dress, will be wearing classic dark suits.

Perhaps this dress would be a good option: http://www.tmlewin.co.uk/product.aspx/WomenSuit/WomenswearSuitsSkirts!35972