The Daily Telegraph is this morning reporting legal obstacles to a free trade agreement along the lines of the suggested Canada Plus model
http://www.telegraph.co.uk/business/2017/10/31/theresa-mays-brexit-strategy-hits-legal-minefield/
"Theresa May’s quest for a “Canada plus” deal in Brexit talks has run into serious legal obstacles and may ultimately prove impossible, risking bitter disappointment and a political crisis next year as reality hits home.
Trade lawyers and European officials warn that even if the EU wants to grant the UK a bespoke deal that preserves unfettered access for goods and services, it cannot do so without granting parallel concessions to Canada, South Korea, and other countries that have free trade agreements. This would open up a Pandora’s Box.
“It is a minefield. Financial institutions are fully of aware of the problem but we have not yet heard anything from the Government,” Chris Bryant, an EU expert at lawyers Brewin Leighton Paisner."
The rest of the article is behind a paywall but the gist is that lawyers are saying if the EU grant us more favourable terms, clauses in other trade agreements mean these would also have to be upgraded to better terms without the EU getting anything in return and this could cause ratification problems.
The article goes on to say that assuming the difficulties are chiefly political and can be overcome by political finessing is a recipe for a rude awakening next year.