I still believe in pacifism, Florence.
That hasn't changed.
But I'm as capable of killing as the next man.
'Foyle's War' - "A Lesson In Murder"
This will be a rather simple splainin as to where the Doctor obtained the DNA he used in his next regeneration.....
In June of 1940, DCS Christopher Foyle had three deaths to investigate in Hastings - the suicide of a pacifist named David Beale while in police custody, the bombing death of a little boy named Joe Cooper, and the shooting of wealthy Lawrence Gascoigne.
One of the suspects in the deaths of Joe and Gascoigne was a friend of Beale's named Theo Howard. He loved his friend dearly and realized that because of David's death, he was capable of murder if given the chance. He eventually enlisted in the military even though he knew it would disappoint David's wife Florence.
By January, 1941, Theo Howard would be in uniform but probably not yet stationed overseas. On January 20th, a Monday, he could have been given the chance to go out on the town in London and he found himself in a small nightclub. While there, a strange man in a leather jumper got up on the stage and started asking questions that seemed quite odd. (That's because it was the Doctor, and he thought he was addressing an audience from the new millennium, not from the era of World War II.)
(The Doctor takes the woman's place at the microphone.)
DOCTOR: Excuse me. Excuse me. Could I have everybody's attention just for a mo? Be very quick. Hello! Might seem like a stupid question, but has anything fallen from the sky recently?
(Silence, then laughter.)
DOCTOR: Sorry, have I said something funny? It's just, there's this thing that I need to find. Would've fallen from the sky a couple of days ago.
(An air raid siren sounds. Everyone starts to leave.)
DOCTOR: Would've landed quite near here. With a very loud
MAN: Quickly as you can, down to the shelter.
We don't get to see the Doctor leave that nightclub in the episode of "The Empty Child". But it is pozz'ble, just pozz'ble, that on the way out, in all of the confusion and fear to reach the closest bomb shelters, the Doctor would have bumped up against Theo Howard.
And that's all that it would take.....