The agreed transition period for the UK leaving Europe is to end on 31st December 2020 @ 23:00 (GMT). After this, many new systems and procedures will go into place for any and all haulage entering/exiting the UK into the EU.
To help operators prepare for this, the RHA has issued a checklist which outlines steps you must take NOW and those you can sort out later down the line. Obviously this information may change, and these cannot cover all aspects that are to come with this monumental change to how we operate a border between the UK and The EU, but it is a strong foundation to get yourself prepared from.
This checklist can be read here.