And conversely, a US court doesn't have jurisdiction over a UK watchdog. So I'm not sure what this lawsuit hopes to achieve, other than reiterating what you said.
The UK does not have jurisdiction over foreign companies. They only have jurisdiction over UK companies, which may include UK arms of multinationals.
The UK does have jurisdiction over the internet within the UK and lawmakers where aware that there is little they can do to compel foreign operations to comply. That's why the OSA gives Ofcom powers to disrupt these operations by compelling other businesses to take action.