There’s a serious problem with this exhibition at Tate Britain – and it’s not the politics. Yes, there is some hideous imperialist propaganda on show and, yes, some of the artefacts were almost certainly looted. But what finally does for this display of four centuries of colonialism is the paucity of artistic accomplishment. Six rooms containing at least 100 exhibits – including works by such luminaries as Van Dyck and Reynolds – manage only three masterpieces between them: an amazing William Blake vision of Nelson confounding Leviathan, a wondrous portrait of TE Lawrence by Augustus John and, best of all (and the last picture in the show) a 2009 mixed-media poster by the Singh Twins entitled enTWINed. It’s almost worth the entrance fee just to see those three.