Without trade, every extra unit costs what the country itself gives up
Without trade, a country that wants more of a good must face its own opportunity cost. A UK worker can make 12 aircraft parts or 6 rolls of cloth, and a Vietnamese worker 2 parts or 4 rolls. With ten workers each, split evenly, the UK has 60 parts and 30 rolls and Vietnam has 10 parts and 20 rolls. Every extra part Vietnam makes costs it two rolls of cloth.
Predict first
The UK can make more parts and more cloth than Vietnam. If the two countries move workers between the goods, can their combined output of both goods rise?
Yes. A part costs Vietnam 2 rolls and the UK half a roll, so moving Vietnam's workers into cloth and some UK workers into parts raises the total of both goods, with the same workers as before.