I'm not British, but I've worked for several British companies and this topic has come up in conversation when I complimented someone on their black shoes with a Navy suit, as it's a combination that I never thought to pair (I hate black shoes).

My understanding is brown shoes are considered to be sporty or informal Britons. It's something you'd wear to go hunting or on a picnic, never to a work. So I think wearing brown shoes with business attire is seen almost like an American wearing hiking boots.