"The pint was normally 16floz in the 18th century. The Imperial (20floz) pint became the British standard in the early 19th century. Was this to make it closer to the half litre?"

I don't think it is, Dave. The American pint is 0.473 cl; the Imperial pint is 0.568 cl. I wouldn't think that either pint measure was influenced by the litre.