Filing as Self-Employed (when it is a 2nd job)

Hi! Soooo, throughout the year I make a pittance as a self-employed copy-writer/editor/etc. It will be under £5000 for the year (well under).  When filing as self-assessed, do I also need to include all of the information from my "real" job (when I PAYE with that one)?
Argh. Let alone how this will complicate my US taxes this year!

Hi! Soooo, throughout the year I make a pittance as a self-employed copy-writer/editor/etc. It will be under £5000 for the year (well under).  When filing as self-assessed, do I also need to include all of the information from my "real" job (when I PAYE with that one)?
Argh. Let alone how this will complicate my US taxes this year!

Yes. If you file a tax return, you'll need to include PAYE income too.

For the US tax return, file Schedule C, and use the totalization agreement so you don't need to pay SE taxes there. It's worth changing your Accounting Year End here to 31 Dec to make it easier to file the Schedule C.