Upvote:1
You are "Self-employed", and you proof this with the contract or/and payments you receive from you "employers" (or bank account where you have regularly payment from the company).. If you should notify somehow your country about your self-employment (taxes, social security, etc.), you will get additional official documents.
I think you should attach some contract/email where there is a long term commitment from the company (e.g. you are committed for a 18 month project, or so). Also if it is not 100% sure, it has some guarantee of your income then most of self-employment jobs.
A warning: you should do an research on internet about your company: there are many scams against foreigners (also because it is difficult (or expensive) to go to court remotely (and foreign people knows much less local laws, and scammers tend to misrepresent it).
Upvote:3
Since the company is covering all the costs of your trip, I would ask them also to cover the costs of getting a UK solicitor who specialises in this area to prepare your application. We have far too many questions around here where people's immigration records have been ruined for life because their then-current employers played fast and loose with their visa status.
You're travelling as a professional; do it professionally. If your employer won't do this, it is a giant red flag and you should consider your position very carefully.