Are 'on sale' and 'for sale' the same in meaning in British? Are they different in American English only?

I don't know about British English. The usage there may be somewhat different. In America, "on sale" usually means at a reduced price. "Originally $39.99, now #29.99. Sale ends Monday." "For sale" means that the owner wants to sell the item, like a house or an automobile or other privately owned property.