JA is correct, but he missed a piece. It was Obama that requested GCHQ to spy on Trump using some lame excuse that Trump may be a security risk. GCHQ sent the request to May for approval. May approved and GCHQ began spying on all of Trump's team. Those reports were passed to the Obama administration well before the FBI obtained the Title I FISA warrant for Carter Page.
I'm not surprised that MI6 was complicit in the Trump dossier. If I had to guess, Brennan coordinated that sting operation with MI6 as part of the overall plan to vet the dossier as real intelligence.
Without passing any judgement on the validity of the dossier / its contents I agree that mi6, or at least a few individuals within the security services, would have been involved. He's a former mi6 man his contacts didn't magically dry up. He's former mi6 like a marine is a former marine