August is a poor time to visit Paris - many restaurants are closed. In London my instinct for such an occasion for a first-timer to the city would probably be The Connaught. The Goring is also a good choice and, in August, it has that touch of true luxury - a garden.
In Paris I always tend to veer away from the big glitzy places run by chains. The Crillon was always my favourite of the luxe-palaces and it's in a truly classic building and location. I also love the Pavilion de la Reine in the most beautiful and romantic square in the city.