Calls from people to Iains to give a link are fine, if they provide links for their statements as well.

It appears Iains may be relying on the following from Economics help:
Disadvantages of a customs union
•A country can't negotiate separate deals because there is a common external tariff. This reduces economic and national sovereignty. Critics of the EU argue it has meant the UK has experienced higher food prices and reduced the welfare of low-income consumers who face higher prices.•It is worth noting – Deals can be struck if they respect the common external tariff. Turkey is in a partial EU customs union so is free to negotiate deals in other areas like agriculture where it is not bound by its customs union arrangement.

•A country cannot give preferential tariffs to a declining industry. For example, if UK steel industry was having difficulty the government might like to put tariffs on imports to protect domestic sales, however, in a customs union you can't choose to have this separate tariff.
•Trade diversion. A common external tariff can lead to trade diversion. For example, when UK joined EEC, it had to raise tariffs on imports from the Commonwealth. This means higher prices for imports of butter and lamb.