The Courtauld Institute of Art, including the Courtauld Gallery, has an important collection of old master and impressionist paintings
It is situated in Somerset House, a large building situated on the south side of the Strand in central London, overlooking the River Thames, just east of Waterloo Bridge. The building dates from 1776–96.
The website is here
The Gallery is a lot quieter than the other galleries in Central London and offers the opportunity to closely study great works unhurriedly in a beautiful and tranquil setting.
The admission fee is £ 5 but Mondays are free.