In order to provide certainty for businesses and bureacrats, to a layman it appears practical for Britain to carry out trade talks with the European Union in parallel with the divorce talks.
Yet the consistent message I hear through the news media is that progress must be made on the divorce talks before the trade talks can begin. The divorce issues that seem to have been most difficult over the last few months seem to have been:
- the rights of EU citizens living in the UK
- the land border between Ireland (the Republic of Ireland) and Northern Ireland
- the 'Brexit bill', the contribution that the UK will pay to the EU budget above its annual commitments to 2019
I have heard various sources, including the leading EU officials, and German government ministers, insist these divorce issues are agreed before trade talks begin. Meanwhile the British negotiators seem to accept this.
Not only do I not know what the reason is, I don't even know whether the reason is political, a negotiating tactic by one or more party to the talks, technical (for some legal or bureaucratic reason), or logistical (such as appointing a sufficient number of officials to conduct multiple parallel negotiations.)
In searching for the reason, I found a BBC article that says the Irish government has asked for progress on the land border before trade talks begin. But I haven't found any other solid, neutral analysis of the reason(s) that the talks are organized sequentially instead of in parallel.
Update these related answers describes the content of the talks in 2017, but not the structure.