Let me try and put things simply in a way that some may understand.

If a block of countries buys 1000 of one product it will get a better price than if it only buys 500 of that product. Economies of scale.

Thus if the EU buy 1000 of a product for £100 each they get a good deal.

However if one country drops out of the EU and the EU only buys 900 of that product the price will increase slightly, for example to £105 per item.

If the one country that has dropped out of the EU wants to buy only 100 of that product the cost to them will be higher because they are not buying as many. The cost to them may be £120 per item.

Very simple economics that anyone should be able to comprehend.