The main tension between London and Brussels seems to stem from a fundamentally different approach: for the UK government, Brexit is a political process, for the European Commission a legal-administrative process.
One might also think that the difference between principle-based Common Law and bright-line based Civil Law/Continental European approach shines a bit through, but I would put more weight on the political explanation. From the viewpoint of the EU, it is very
simple: the EU cares primarily about the interest of its remaining 27 countries and their people (important if you think about the issue of the Irish border). The EU has no business to solve internal policy conundrums in the UK or help the UK government
to resolve problems stemming from Theresa May’s mistake of drawing red lines for Brexit before thinking through the implications. For the UK government, on the other hand, this process is as much about managing expectations with its electorate
and internal Tory critics on both sides as it is about negotiating with Brussels. It is more, the UK government seems to negotiate more with itself than with the European Commission. Therefore it is not surprising that even though it was the UK government
that started the divorce process, it is the European Commission that has produced a first draft of a divorce document.