Logically therefore, a Brexit without CU Membership would lock UK firms out of the EU.
Either:
a) The EU is a protectionist fortress and so leaving it will sever UK exports into Europe;
or
b) The EU's protectionism is no big deal, so leaving it won't see huge competitiveness gains.
If the EU protectionism is so stark, won't big business in the EU27 fight tooth and nail to maintain it once it's restored vis a vis the British?
It provides sufficient advantages to basing in the EU28 that when choosing between two otherwise approximately comparable value propositions, firms will opt to do so.
The harder the Brexit the fewer such nudges will continue to push firms wishing to trade into Europe to base their operations in the UK.
The string of UK businesses pulling up stumps would suggest there's skepticism on that score. /end