On 17 December 2017, the UK House of Lords published its report on Brexit; Sanctions policy. In its wisdom, the Lords have advised that the UK maintain a closely knit multilateral framework with the EU and the U.S. regarding sanctions (hopefully also for export controls??).
The Lords state the obvious, as in the post Brexit world, the EU, UK and the U.S. essential interests and threats will not fundamentally change. The UK aligning itself with EU sanctions as a third country would not permit it to influence the design of sanctions, and formal inclusion in EU meetings rather than information cooperation would be the most desirable approach. I would argue that it would make sense that the UK somehow remain within the EU’s CFSP framework.