When the British recipe calls for plain chocolate do they refer to cocoa powder, chocolate syrup, cooking squares, or Nestle style chips? Is it unsweetened, semi-sweet, or bitter?
British plain chocolate would be broadly equivalent to the US bitter-sweet chocolate.
Bitter or unsweetened dark chocolate has high cocoa solid percentage and little to no sugar added.
Bitter-sweet has about 70/30 cocoa solid/butter to sugar ratio.
Semi-sweet has about 50/50 cocoa solid/butter to sugar ratio.
The closest US equivalent to British plain chocolate is semi-sweet. If a recipe calls for, say, 5oz of plain chocolate, it will mean chocolate in bar form (unless chips are specified of course).
Obviously if the chocolate is for melting it doesn't matter what form it comes in.
Cocoa powder and chocolate syrup mean the same thing in Britain as they do in the US, incidentally.