UK does not fit well into the grouping as it is not located in the Pacific and it is in the interest of Malaysia/Brunei and those in the Americas to keep UK out of the CPTTP.
I'm grateful for you to add some country details, as it helps greatly.
I'm still not sure why those countries would be vetoing the UK joining. It is not situated in the Pacific but it has the fig-leaf of territory (Pitcairn Islands) to avoid any precedents being set. More importantly it is another major world economy (alongside Japan). Just because the UK mainland isn't in Asia doesn't mean there couldn't be increased trade. We can all see how much trade China does even with countries on the other side of the world, and if friction with EU trade continues UK companies will look to work more closely with other countries.
The UK would also be the first permanent member of the UNSC in the organisation. CPTPP isn't a political union but UK membership could give existing partners greater ability to lobby the UK on foreign policy and defence matters. That's nothing to be sneezed at.
Whilst taking into account all of the above, the UK is not a current/potential superpower, nor does it see a need to assert control over other countries for its own security. It also does not have any particularly heavily-subsidised or super competitive manufacturing/agricultural bases that could take away jobs from CPTPP members. That makes the prospect of UK membership relatively benign.
Away from the fraught endgame of the Brexit negotiations, there has been positive news in UK trade in the last few weeks. International Trade Secretary Liz Truss added to the growing portfolio of UK trade agreements by securing a deal with Mexico on 15 December, following similar deals with...
policyexchange.org.uk
This article from December pointed out that at least 7 of the 11 CPTPP members supported the UK joining, which suggested that at least some of American states were included.
It is correct that the UK is not currently negotiating trade deals with Malaysia and Brunei. However, the UK has pretty strong relations with Brunei, and my understanding that these were expected to continue as we left the EU.
Ties with Malaysia may not be quite as robust, but I've only seen positive news about further engagement with London and no obvious political or economic rivalry. Also since the UK has left the EU there would be scope to discuss renewed palm oil imports, with the UK previously having to follow the EU ban.
In short, even if the UK was not the most optimal new member for CPTPP, it would be a benefit. There's no hard cap on the number of
people states that can be in CPTPP, nor does the accession process for one country stop others from joining at the same time.
When Vietnam assumes the chair in 2026, will we see real urgency to add UK (as it is in Vietnam’s interest to add UK).
If it took until 2026 then so be it, but I think it could be sooner than that, not least with Japan's open support for the UK joining and it taking over leadership this year.
CPTPP remains the centerpiece of any future US reengagement and Japan will see to it that it’s terms are not supplanted by a wholly new framework.... The CPTTP terms for new members are take it or leave it (with further concessions to US the only exception).
In that case I think the chances of UK membership are better than I originally thought, because either the UK will agree to CPTPP's position or it won't. If there aren't negotiations as such, rather a set of things to agree to, it makes the process much simpler. Given that the UK has already got FTAs agreed with the majority of CPTPP members and there being a reasonable chance of getting them agreed with Australia and New Zealand in the next few years (Boris will want more political wins and won't worry about giving some concessions), the distance between the UK and CPTPP could end up being fairly low irrespective of the application to join.