The only cost cut from Berlin is the 10% import tax, which I suspect is a net negative to Tesla when you factor in the cheaper cost of production in China.
The shipping argument isn't clear cut, I do not know how much the 28 day charter is from China to the UK is but it is divided by let's say 5,000. The RoRo port is 1-1:30 hours from Giga Shanghai, this matters because trucking the cars is a big bottleneck and per mile the most expensive bit of shipping the vehicle. These are hypothetical numbers but let's say for the trucking to the RoRo port, it costs Tesla $50 a vehicle as it's a short enough trip. Now, the 5000 cars get on the boat, let's say it costs $500 per vehicle to ship them to Southampton. The landed cost in the example is $550 per vehicle/$2,750,000 for 5000 vehicles.
For a Berlin vehicle, you've still got to truck the vehicles to Bremerhaven (5-6 hours from GF4) for a RoRo to Southampton (I'm guessing). Now it's Germany, we already know costs will be x higher than China and now we've got a 5-6 hour truck that only carries 8 cars. So Let's just say it costs Tesla $300 per vehicle for the trucking part, then the RoRo part costs let's say $100 (I think that's low). The landed cost in the example is $400 per vehicle/$2,000,000 for 5000 vehicles.
I don't expect those numbers to be accurate, they are rough and based on my experience with importing. Tesla has a delivery charge of £800ish so I suspect I'm not far off considering you then have to include the costs after the vehicles have landed. Trucking the vehicles would only work on mainland Europe, as soon as there's water involved, it's best to just get it in the water as close as possible to the production site and delivered as close as possible to the final destination. There is the possibility that rail is used but that seems unlikely. There's a reason all the boats from China bypass Italy and France first, it's cheaper than transporting them on land.