Really not sure what to advise regarding the dress code, as long as you are comfortable in whatever you are wearing. In the mid 1970's I sometimes used to go to the Dorchester (just across from Hyde park) with my brother, who worked in London, for their prime rib buffet - I remember chefs carving off huge slices of meat, other joints as well I recall. All washed down with Trader Vics cocktails beforehand, and nice claret to accompany the meal.