A UK Japan Economic Partnership Agreement is an interesting one for two reasons;
This is really a three way deal between the UK, Japan and the EU. The supply chains for Japanese companies based in the UK (Nissan and Honda re the elephants on the room here) are predominantly EU based. The value and success of a UK-Japan agreement is very dependent on the success of a UK-EU agreement.
The second issue is the EU-Japan agreement has 'Most Favoured Nation' clauses which state that any concessions offered to a third country, the UK in this case, must also automatically be offered to the EU. The best possible deal the UK can get from Japan would match what the EU can get (whether the EU would necessarily accept/implement a MFN concession is a different thing of course)
This article explains this well -
https://blogs.sussex.ac.uk/uktpo/202...mental-issues/