So then, the Agency promises you promises and the Company "hires" you on a "contract". Which means you are not "working" for either. You work either for your own company or the Umbrella, the Agency is your pimp and the Company is organizing a party. You are the rentboy waiting for the customer with a tub of vaseline ready to get reamed.
Oh, the Company doesn't win the tender, so no vaseline.
However, the other pimps that were calling you, someone won the tender so look out for the same/similar job popping up now that they actually figured out they need people who do XxX and not xXx
The main tax office test in the UK for a contractor is that "you do not have a boss" and you make "youre own decisions". Basically, they can't make you wear a company uniform, you're a mercanary. A "technical consultant"- the Agency (pimp) has a "handler" (madame) which is your contractual boss. But you're there to do a specific tangible piece of specialist work... So if you're hired for screwing them proper, you won't reach around

So being a contractor is a lot like being a hairdresser...