While browsing the London 2-4-1 offers I discovered an upcoming special Van Gogh exhibition at the Tate Britain. It runs from March 27 through August 11. Although in theory a 2-4-1 deal should be available, in my experience special impressionist exhibitions tend to be extremely popular and subject to sell-outs, probably especially on weekends and toward the end of the run. I think this makes relying on the 2-4-1 deal to cut the cost of this very expensive exhibition (£22, £20 for seniors) quite risky, especially for those planning summer trips to London.
I recommend that those for whom this exhibition is a top goal monitor ticket availability on the Tate website and be ready to purchase at full price if time slots around the travel period are beginning to sell out. The tickets are already on sale.