So how did British English come up with a double tee?
Puzzled.
I suspect (although I have no proof) that it has been modelled on the double
z in the word
pizza.
They are, after all, basically the same word, although one is Italian and one is Greek.
Many Italian restaurants started opening in UK in the 1960s, and European food became more popular after we joined the Common Market (aka the EEC / EU) in the 1970s.
As for pronunciation, I've always pronounced the word as a homophone of
peter, although I have heard some people say it as
pitter.
Finally, according to the COD, both bread and bird are pronounced in the same way - like
peter.