1) You cannot be selective when you have a free trade agreement - the Brixiteers think they can do sectoral deals for cars and perhaps pharmaceuticals and possibly agriculture. Sectoral deals are not allowed according to the WTO rules a free trade area must cover "substantially all trade" only then can it be admitted as an exemption to the WTO principle of non discrimination (that is we have tariffs against the US and Japan and not with the remaining EU27).
2) If you remain in the customs Union then you have to adopt the EU common external tariff, this means that you are not then free to do free trade deals with other countries. If you did so then that would undermine the common external tariff. So you are either in the customs union for good or you are not.
3) The EU 4 freedoms that is freedom of movement for goods, services, labour and capital are "indivisible" if you want free trade in goods and services then you have to allow free movement of labour and capital. If you restrict free movement of labour to control inward EU migration then you lose access to the single market.
4) You cannot say you wish to end the jurisdiction of the European Court of Justice and remain in the single market or customs union. The ECJ is the arbiter of the CU and the Single market so once you rule out its jurisdiction over trade and free movement of factors of production then you cannot be in either the CU or single market.
5) If you do not accept EU regulations then it becomes very hard to trade with them where you do not meet their regulatory standards. This means that if UK deregulates then it can only sell its goods and services into the EU if the standards are deemed to be "regulatory equivalent" and that is an ongoing process and means keeping abreast of changes in the EU regulatory environment.
There is a really nice video below where Michel Barnier makes these points in a crystal clear manner. The Brexiteers need to get up to speed very quickly or they will be dragging the UK economy and its citizens into a very hard Brexit. The clock is ticking and they need to get their act together ! Unfortunately, they are so out of touch and stupid that I can only foresee them being forced to understand the reality when it is too late. However, do watch the UK opinion polls as the costs of Brexitbecome clearer and clearer I expect the UK opinion polls to change in favour of remaining and it will be very hard to push some of the Brexit bills through parliament and the House of Lords. A second referendum may be the
only way out of this mess eventually.