Most experts on trade matters are clear that a bespoke Comprehensive Framework Trade Agreement (CFTA) between the UK and EU can be expected to take at least five years to negotiate after UK exit. For example:https://blogs.sussex.ac.uk/uktpo/2017/12/15/magic-realists-and-economic-realists/.
The Stage 1 negotiation process showed that the EU calls the shots in this process. It has confirmed that the purpose of the impending stage 2 exit negotiation phase is to secure full agreement on the divorce deal, including the outlines and parameters of the future trading relationship between the two parties, the detailed negotiation of which will follow later.
Insofar that the UK chooses to pursue a bespoke CFTA in line with current government expressed intentions, it is most unlikely to be operative by 2024, at best.
Assuming that the UK does irreversibly leave the EU in March 2019, subsequent to a UK Parliament vote, but then continues with de facto single market (SM) and customs union (CU) membership regime for two years until early 2021, some of these experts, as above, have pointed out that would mean the UK falling off the cliff in mid-2021, leaving a hiatus until a new CFTA is ratified.
It follows that transition period involving continuing de facto SM and CU membership or, alternatively, postponement or extension of article 50 until 2024 or when the CFTA actually becomes operative, is required.
That, however, would be unacceptable to the conservative hard Brexit wing. They could bide their time until the Brexit divorce is legally sealed in early 2019, and then agitate for ‘real Brexit” without continuing SM and CU de facto membership: in other words, a disorderly exit.
Meanwhile it appears that the May cabinet has authorised an approach where the UK will seek from the EU, a transitional deal where the benefits of continuing CU and SM membership are retained, viz: frictionless trade in goods without tariffs and without border customs checks, but with some regulatory divergence in key sectors to the UK interest, as well as, say, exit from the Common Fisheries Policy: the ‘let us have our cake and eat it’ option.
It is almost certain that this approach will be rejected by the EU next spring; perhaps that will serve to further clarify minds.
The prospect of de facto five year SM and CU post 2019 transition period-at least involving continuing FOM, EOJ jurisdiction, and membership fees, does not appear yet to have been accepted or understood by Labour, at least in public discourse or in policy terms. Perhaps that will change when the fixed parameters of the stage 2 negotiations, as set by the EU, become even more visible by March 2018, as above.
By then it could be clear that the real choice is either a disorderly exit or a status quo where the UK continues as before until 2024 or later, but without influence; that is, unless, the UK seeks to postpone Article 50 divorce with EU assent.