18 months is achievable if we put our mind to it, especially if we agree transitional arrangements.

Normally with third party negotiations like Canada etc we are starting with two completely different sets of arrangements and laws etc and trying to negotiate where to merge them to be the same and who should give way where. This isn't the case for the UK, we already have the same laws etc where relevant to the EU as we are already a part of the EU. Instead in future we may divert but if we sign agreements not to divert in critical areas X, Y and Z that are necessary for the free trade agreement then to simplify things that is the end of the matter. Much easier.