Apart from that "up to" 400 MWh/year isn't very large by today's standards, this seems to be a plant to produce batteries, not cells.

The difference is relevant because of FTAs' rules of origin. If you want your cars to benefit from an FTA's zero tariffs, typically a local content of some 55% by value has to be met. For comparison, pre-Brexit, a typical UK made internal combustion car had a local content of about 40-45%. To build an EV in the UK that meets a 55% threshold is practically impossible if the cells have to be imported. Most of the raw materials to make these have to be imported either way, which is bad enough, so you'd at least want cell production, and ideally most of the chemical supply chain, to be domestic.

That is, the TCA allows for bilateral cumulation, i.e., content from the EU counts towards the 55% that is required for EVs from 2027 onwards, but only half of UK car exports go to the EU. (It also comes with extra local content requirements specifically for batteries in EVs, and they're so high that meeting them is an ambitious goal even for an EU based carmaker with its own cell production.)