The British stiff upper lip is more relaxed, but on social occasions the rule is to observe and then go with the flow. If you're visiting a family home, a gift of flowers is welcome; if you're eating at someone's home, you could bring a bottle of wine, perhaps, and maybe some candy for the children -- but not necessarily all three. British people will shake hands on greeting old friends or acquaintances; female friends may greet each other with a kiss on the cheek. In Britain, you can never say please, thank you, or sorry too often; to thank your host, a phone call or thank-you card does nicely.
In business, punctuality is of prime importance; if you anticipate a late arrival, call ahead. For business dinners, it's not assumed that spouses will attend unless prearranged, and if you proffered the invitation it's usually assumed that you will pick up the tab. If you're the visitor, however, it's good form for the host to pay the bill. Alternatively, play it safe and offer to split the check.