The British restaurants refused from savage etiquette and changed over to liberal approaches to their guests' clothing. It is mostly due to the rivalry reinforcement, and under its conditions, the public catering establishment prefer to take no notice of their clients' clothes.
In the course of decade, many establishments like that required men to wear strict suits with tie or bow tie.
Jeans or any "casual trousers" were not allowed. As for women, there were definite restrictions regarding the trouser suits.
By the way, the name "dress code", as the world goes, has appeared more than 100 years ago just in London for the first time.
On the safe side
Since "dress code" is going on to work in any other places after its cancellation in British public catering, here is the decryption of the more widespread terms, accepted, for example, for the invitation cards for public and business events.
White Tie - the modest formal code. Man should be dressed in a tailcoat with a white bow tie, jacket also in white; for a woman - an evening dress "till floor", gloves, heeled shoes and a tiny purse. Costume jewelry and naked arms are not allowed. Possible events: president's or ambassador's banquet, wedding reception, ball, prize presenting.
Black Tie or Formal - man in a jacket, woman in an evening dress "till floor" or in a cocktail dress. Jewelry is allowable. Possible events: official evening reception, for example, Christmas or New Year's banquet, wedding, opening night in a theater.
Black Tie Invited - man in a jacket, woman in a cocktail dress, a long dress or a stylish suit. Possible events: banquet in an elegant restaurant, corporate banquet, family occasion. The organizing 
