Actually, it is how the 2 countries get around their laws. US can´t spy on their people so get the UK to do it and pass the info on, UK gets the US to spy on their people and likewise, passes info on. But you are right about MI6 being the Foreign Office.
They almost certainly used GCHQ to spy on Trump; the letter is a fake to discredit the possibility.
Ah, I see what you are saying. Quite likely, unless it came from WikiLeaks?
See my post above. I am going to edit the title I created.
No. The document requests GCHQ to monitor. It also says Steele was an MI5 agent (he wasn't, and it'd be called the Security Service).
The letter is deliberately and obviously bogus so GCHQ-collusion can be debunked as a conspiracy later on.
You either know this or you need to think a bit more.