Re: There 'May' come a time
Meeting your obligations to withdraw is just that, meeting the obligation which includes a fiscal one.
The trade agreement can only happen after the withdrawl is agreed for the simple reason that the trade deal cannot be put in place until we actually leave and have third country status. Thats why the stages are Withdrawl, define trading arrangement, sign deal on the day of leaving (if one is done of course)
As to the import at lower or zero tariffs argument (the Minford fantasy) , fine if you are purely a consumer economy but even the economic sage himself says that approach will completely destroy manufacturing in the UK. It will also hammer our exporters, end result will not be pretty.
The financial sector will be really hit hard if by March 2019 we don't have adequacy to handle the data of EU citizens or a deal on passporting, hence all the major financial institutions making contingency arrangements in the EU27 for that eventuality, just look at the doubling in commercial rents in Dublin since the vote, they are loving it.
As for Euro clearing, some of it already takes place in the US, they wont say no to more, plus your comment about reserve currency status is rubbish. The dollar is a reserve currency and almost all of its clearing is in New York so moving Euro Clearing more heavily into Europe won't change the reserve status of a currency.
Automative will be interesting, we don't have the supply chain in the UK to replace the multiple component moves that make up modern car manufacturing. End result if we do become a third country with no deal is a significant impact on the major manufacturers, all of which are fully or partially European owned. Of course the government has given a comfort letter to the Franco-Japanese owned Nissan and will probably have to do the same to BMW Mini, Tata Land Rover, Honda, Peugot (now owners of Vauxhall) etc. You or I won't get a comfort letter telling us the UK taxpayer will make up the difference, we will be the ones paying it. But even with those kind of subsidies/guarantees etc outside of the luxury producers you will see a big contraction, its already happened with investment plummeting since 2015 when the referendum was announced.
The real irony I have saved until last, now we are leaving we won't have a seat at the table and as a third country we will have to adhere to ECJ decisions and jump through all those hoops just to trade with the largest market on our doorstep. Its worth remembering the impact that the ECJ ruling in Schrems v. Facebook referred by Ireland had, even the land of the Trump has to bend to the ECJ if it wants to trade with one of the other big three.