It depends what you want to see. The Imperial War Museums in London and Manchester are good, and free. Most regiments have a museum, and these can be of widely varying standards of display. Bovington camp in Dorset has a tank collection, then there's the RAF museums at Hendon and Cosford. Most local museums have a decent 'home front' type display but phone and check in advance - permanent displays at Manchester Sci & Industry museum / Glasgow people's Palace / York Castle give a good idea of what homes looked and felt like.