It has been taken to a central hub at an undisclosed location, and will now be distributed to hospital vaccination centres around the UK.
The UK has ordered 40 million doses - enough to vaccinate 20 million people.
England's deputy chief medical officer said the first wave of vaccinations could prevent up to 99% of Covid-19 hospital admissions and deaths.
Speaking to BBC News, Prof Jonathan Van-Tam said that would be possible if everyone on the first priority list took the vaccine and it was highly effective.
He said it was key to distribute the vaccine "as fast" and at the "highest volume" as possible, but he acknowledged there would need to be some flexibility in the list.
The Pfizer/BioNTech vaccines are made in Belgium and have travelled to the UK via the Eurotunnel.