Lenny Henry (in-universe)

As "Sir Lenny Henry", the apparent 9.5th Doctor, or 9.5th Comic Relief Host, was a presenter for Comic Relief.

While getting ready in his dressing room to host Comic Relief, he began feeling ill. Regenerating in front of a baffled production assistant, he turned into a new Doctor. (NOTVALID: Lenny Henry Regenerates into David Tennant)

Behind the scenes
Although he is called "Lenny Henry" in dialogue, Henry's character in the minisode is identified as the "9.5th Doctor" in the title of the YouTube upload of the regeneration scene, and as the "9.5th Comic Relief Host" in the title of the Facebook upload of the full minisode. The YouTube upload simply used the actor names, Lenny Henry and David Tennant.

Accepting the notion that Henry is to some degree playing the Doctor in the skit, this marks the second time that he has played the Time Lord, as he previously portrayed a parodical Seventh Doctor in an untitled sketch for The Lenny Henry Show. However, there is nothing to connect the two portrayals, making this similar to Jim Broadbent and Richard E Grant's similarly distinct portrayals of non-mainline Doctors.