Jodie Whittaker was announced as the Thirteenth Doctor in July 2017, after what seemed like an age after Peter Capaldi confirmed he’d be leaving Doctor Who; now, Jodie’s revealed that we might not have found out even then, if she hadn’t have needed to record her regeneration scene so soon!
We saw the Twelfth Doctor regenerate in the Christmas special, Twice Upon A Time, and Whittaker apparently recorded the final scene of that episode all the way back in July too. She told Doctor Who Magazine:

“As soon as they made the announcement I was in the studio. I think the announcement happened so they could get me to the TARDIS set to do the regen without it being leaked. If I’d been seen in Cardiff, being taken to Roath Lock [Studios], it would have been very obvious that I was playing the part! So they announced it the day before.”

She also says that she knew nothing about the rest of the episode, instead solely being given that lone final page:

“There’s not a lot of prep you can do for that! I didn’t read the rest of the episode because I was only given a page to go from, which was the moment that I become the Doctor. In a way, I had one of the easiest jobs, because all the hard work had been done by Rachel [Talalay, director], the art department, and the rest of the crew. I’d never stepped on that set. Almost everyone else who was shooting that scene had been working there for months and months, but I was there for a matter of hours.”

We can only presume she recorded the adjoining scene – in which she plummets out of the TARDIS – against a green screen on the same day.
So while we were enjoying the summer heat (that is, if you, unlike me, enjoy the heat and don’t start to boil and shrivel and weep), Jodie was immersed in a sequence for Christmas!
Filming for Doctor Who Series 11, with new showrunner Chris Chibnall, continues, and the run of 10 episodes is due to air on BBC1 and BBC America in Autumn 2018.