Despite the official looking document he is holding, anyone can issue such a document themselves - it is not handled by a government office, but several companies uses official sounding names to issue documents that look official.
You technically can just start using a name - a deed poll document just makes it easier to get people to accept that the change is real. Some entities may demand that you "enroll" the deed poll document with the courts - basically, just submit it to the Royal Courts so that it gets put in the official record. This is not a legal requirement at all, but just a way for those institutions to ensure that you're not just creating an identity that you have no intention of using in other contexts by putting it on record somewhere searchable.
Technically a deed poll is just a signed declaration without a counterpart where the party states an intent, and not specific to a name change. In the case of a name change, it's just a statement of intent to use the new name and cease usage of the old name.
It's not what it first seems -
https://www.imdb.com/title/tt5824910/
It's explained in the doco but "Cnut Sweynsson, known as Cnut the Great or Canute, was king of Denmark"