BBC Your Paintings is an initiative which 'aims to show the entire UK national collection of oil paintings, the stories behind the paintings, and where to see them for real'. They're doing a good job of it, with 77 000 paintings already online. Some of the images aren't as large as those on some galleries' own web collections, but it's a brilliant resource.
It's particularly great for finding out where you can view paintings by a particular artist. For example, here is the UK's collection of Sargents (or at least the ones that have been digitised so far). And this Portrait Study for Head of Moses can be found at the Derby Museum and Art Gallery (image copyright of the museum):