Iains isn't "relying" on anything otherwise he would have humiliated everybody by putting something up (which is what he tries to do) by putting it up - he made the definitive up in desperation on the spur of the moment
Do you think double posting your tortured abuse of the english language adds clarity? Think again!

"Take a customs union on its own first. A customs union would create far less friction to trade in goods. It would be easier to keep vital manufacturing supply chains intact and it would obviate the need for the customs part of the Irish backstop, as long as agriculture was included. But it would also mean that Britain could not operate an independent trade policy in respect of goods, because we would be obliged by the EU to operate its external tariff rather than vary our own in bilateral trade agreements with non-EU countries.
The biggest shortcoming of a customs union, however – and it is a major shortcoming – is that it does not in itself address the issue of regulatory alignment between the UK and the EU that single market membership gives us. This is the principal guarantee of the barrier-free trade we enjoy currently with the EU and why every alternative to membership will leave us worse off.
For this reason, many MPs are looking to a deeper agreement with the EU, one they call common market 2.0, which in addition to a customs union would mean continuing membership of the single market. The attraction is obvious: it would keep the UK close to its most important trading partner while fulfilling the mandate to leave the EU. It comes with a set of institutions based on the existing European Economic Area through which the UK could potentially influence (but not determine) EU policy and incur reduced UK payments for market access.
The disadvantages are equally clear. Britain would have to operate EU rules, without a vote or a veto, for both goods and services – becoming a rule taker – making it largely pointless to leave. Free movement of people would also continue. Yes, there is provision for an emergency brake on migration but this would not give Britain “control” of its borders."

A hotel California brexit!