the norm these days for half and half in American bars is >to pour the Guinness carefully over the Bass so as not to mix the two ingredients. This gives you a drink with a golden bronze lower layer with a black black upper layer. Is this the standard for half and half in Britain?

It wasn't when I was younger and I haven't drunk that kind of thing for a long time, so I'll rely on trendier types (Shanks are you there) for local knowledge. Some people used to ask for a pint of "mixed" which was half bitter, half mild. I've heard of Black and Tans (Guiness and mild) but never seen them separated like in the following picture:
http://www.schremppstudio.com/beer.html

According to the Guiness website below.
Q: How do I pour a Black and Tan?
A: Steve Glover, who has tended bar in Ireland, says that the layered Black and Tan is an American affectation that they were happy to do for extra money :-). Otherwise, both beers (they used Smithwicks and Guinness) were simply poured in the same glass fully mixed.
Q: Is there any meaning to the Black & Tan name other than its obvious reference to the colours of the beers?
A: Yes. The first known reference to the expression Black & Tan was in reference to a breed of beagles used as hunting dogs in Ireland.
The term was also used to refer to a a regiment of British soldiers recruited to serve in Ireland after the First World War. They had a reputation for being quite brutal and have been accused of many attrocities against the Irish in the years 1919-21.
http://www.ivo.se/guinness/bnt.html

It sounds like, along with Irish Coffee (Buena Vista, San Francisco), the Black and Tan was invented in the USA.

Here's the song:
Come Out ye Black & Tans!
I was born on a Dublin street
Where the loyal drums did beat
And those bloody English feet
They walked all over us!
But every single night
When me Da would come home tight,
He'd invite the neighbours out
With this chorus:

Come out ye Black & Tans!
Come out and fight me like a man.
Show your wife how you won medals
Down in Flanders.
Tell her how the IRA
Made you run like hell away
From the green and lovely lanes
Of Killeshandra!