>decent suit, no tie = business casual, beyond doubt (at least for the UK)
That about nails it.
Dependent on what image you are looking to portray a navy suit, light blue shirt [button cuffs, regular collar] and some brown loafers is a good look. The lighter you go on the shoes the more casual/fashion forward you become, if you are expecting the firm to sport chinos and blue shirts I'd be tempted to go fairly light tan on the shoes.
Don't discount a pocket square...