Many thanks for the reply. I'm new to the world of self employment so perhaps I have got some of the wording wrong. I'll try to clarify...
I set up and registered a business earlier this year. The business is just myself, I have given it a name so it's myself the individual, trading as "Insert Brilliant Name". I have no employees, I provide the services myself.
If I decide to partner with this other startup and take shares and a director role, I could provide certain ad-hoc services under their name, Company B. I would then like to invoice for these services, from my own "Insert Brilliant Name" business to Company B (the company which I could now be a director in).
Does this make sense and is it a legitimate business practice?