Any delay, though, would be as a result of negotiations on trade between the UK and the EU looking like they aren’t going to be finalised by December: I believe a decision on this must be reached midway through 2020: the clause in the agreement allowing for that is what would be removed.
An early trade deal, or effectively, a trade deal of any sort, is more likely the closer the EU and UK remain aligned in terms of customs and standards. In a sense these negotiations will follow the opposite course to normal trade negotiations: ie they’ll start from a position of alignment and diverge.