Possibly via RoRo ship to Sheerness, possibly road transporter.
When the UK rollout started "we" knew a lot about how cars get from the Fremont factory to Europe from tracking EU distribution for far too long. This leg of the journey is new though so we don't really have any firm idea how it'll happen.
We *think* that the small number of deliveries in June came over by road transporter, but that makes less sense for larger scale deliveries. For these deliveries, it makes sense (at least to me) to do RoRo ships to get them to the UK straight from Zeebrugge (possibly split across two or more UK ports) and then road transporter from there. Using multiple UK ports reduces capacity concerns about ships from ZEE to the UK, local transporter capacity when they reach the UK, and reduces the journey time by lowering the transport time here in the UK.
Looking at Europe again, this would be the "Norway" model of Tesla distribution.
What we really need is for someone to spot them coming in at a port / on a transporter to confirm one way or another.