The UK government has so far provided no detail on what the "implementation phase" of Brexit might involve. In our view, given the serious cliff edge risks outlined in answer to Question 1 above, it may well be desirable for the UK to remain within the Single Market and within some form of customs union with the EU for a defined period, so as to allow both business and government more time to adapt. However, this could prove politically controversial, as it would potentially contravene several of the UK government's red lines. For example, the EU would be likely to insist that the UK continued to abide by rulings of the Court of Justice of the European Union as regards interpretation of Single Market rules. It would also probably insist on the UK continuing to make budget contributions towards the administrative costs of the Single Market and continuing to allow free movement of EU citizens.
An interim arrangement modelled on the Agreement on the European Economic Area (EEA) could provide a compromise solution. In particular, it contains provisions which could allow the UK to enact temporary "safeguard" measures so as to address concerns about immigration in the short term, pending the UK's full exit from the EU (when it will regain full control over immigration). In addition, although EEA-EFTA member states make contributions to the EU budget, they do so at a lower level than full EU members.
It is also possible that the UK could ask for disputes over its compliance with Single Market rules to be referred to the EFTA Court rather than the Court of Justice. There are a number of arguments that the UK could make to persuade the public that the EFTA Court model would make a significant difference in practice and ought therefore to be acceptable for an interim period: for example, its judgments are advisory not binding, the system for national court referrals is likely to result in fewer cases coming before the Court and it might be possible for a UK judge to sit on any panel dealing with cases involving the UK (so the UK would potentially have a greater sense of involvement in the make-up of the court, as compared with the Court of Justice where it only has 1 judge out of 28, with no guarantee that he will hear all UK-related cases).
However, EEA-EFTA States are outside the EU Customs Union. This means that the UK would need to reach a special arrangement with the EU in order to remain within the EU Customs Union on an interim basis. Alternatively, it could seek an interim customs union with the EU, similar to the arrangement which Turkey has (although Turkey's arrangement is not intended to be temporary).
An alternative would be to approach transitional arrangements on a sector-by-sector basis, with the UK only continuing to participate in Single Market arrangements where it felt that it was necessary for particular types of business. Such an approach might be less prone to domestic political objections, but it is not without its own problems. From a practical perspective, it is likely to take considerably longer to negotiate than a "blanket" continuation of current arrangements as outlined above. From a legal perspective, it may be problematic because of WTO rules requiring trade agreements to cover "substantially all trade" in goods and/or services, as the case may be; sectoral deals would be unlikely to meet this criterion (although if the transitional arrangements are of relatively short duration, other WTO members may be prepared to tolerate this as a technical breach of the WTO rules, but not one which they would seek to challenge).