Yes Cosford have their own holding flight, so you'll be moving there the day after passing out. We arrived there and were sent on leave immediately for a week before we started our course. We came back off leave and started straight away so we had no odd jobs to do. Your course could be quite different, depending on how full it is/if they're waiting for people (people who have been back-coursed at Halton) to catch up. Anyway yes you will almost certainly not spend any time at ADF at Halton.
There'll definitely be time, it's great here.
Hedge, I was given my PRTC date and RTC start date the day after my fitness test. If I recall correctly it was about 3 weeks until my PRTC and a month after PRTC I started at Halton. So it's likely to be around 2 months.
Your trade is a strange one though, a few of my buddies had their dates brought forward/had their PRTC and then started a week later, so it could be as little as a month away! It's basically pot luck