Somebody on here (Neptune) claims to work in the DfT and somewhere up thread mentioned it. My guess would be Talgo, as they have not built anything for the UK yet.
Soon enough though this bidding process will be public and we will all know, not sure why it's been behind closed doors this long anyway.
Not sure if it is still the case I believe that Talgo are or where interested in building a factory here should they get a large enough order as it would make more sense, than trying to transport the trains from Spain. This is something that Talgo have had an eye on doing for a long while as per the comment below from the Talgo Wiki page:
https://en.wikipedia.org/wiki/Talgo.
"In recent decades, Talgo has made a renewed effort to expand its presence internationally. During the late 2010s, Talgo made arrangements to establish a new train manufacturing site in the
United Kingdom in response to orders for its trains having been placed by multiple British railway operators.
[9][10] In the early 2020s, the company opted to invested in new manufacturing facilities in India with the aiming of securing sizable orders from across the country's highly-trafficked railways."
I believe there was a comment about this also, when Michael Portillo did a Great Continental Railway Journeys episode in Spain, where he visited a Talgo factory.
Also mentioned here
https://zenoot.com/2019/11/04/talgos-new-uk-train-factory-a-step-closer/
But like CAF and Hitachi, the bodyworks building could still take place in Spain, but with final assembly here in the UK.