There may be a number of reasons why UK PLC is unable to grasp the opportunity you suggest.
Firstly, the failure of BritishVolt and the unwillingness of UK PLC to intervene may act as a disincentive to other potential market entrants. Nissan has invested in both a production plant and a battery manufacturer which seems a more stable platform.
British start-ups may be uncompetitive. As you said earlier the rush to green in the US is being driven by Biden's massive subsidies and incentives to manufacture in the US. The EU is sure to follow suit. There is every chance that the UK will be squeezed out both on price and capital whilst both economic superpowers protect and take advantage of their vast internal markets. It is also possible that British engineering talent and ingenuity will be plundered by those markets for our loss and their profit.
There is a significant force in the UK politic and media that sees the transition to green as a threat not an opportunity. We get a flavour of that resistance on here regularly.
In the midst of this the Tory govt is rushing headlong to burn as much EU derived regulation as possible with little inkling what to put in its place. The further that regulation diverts from that supporting the EU single market the more frictional the trade barriers become and the more unlikely that start up electronic car manufacturers will have a viable business plan. Perhaps we can flog them to Australia and New Zealand.