The best thing to do would be to give your TA centre a call mate.
RG8 forms you're collecting will take a while for the GP to get all the reports, etc, if necessary. So that could take a while. For me, getting my RG8 sorted took 3 weeks after giving it to my GP, although I'm not saying that will be the case for you.
After you have your RG8 forms ready, they have to be sent off and assured that they're fine, then the unit will book you on your RSW. I'm not sure about the medical process now, as I was told it was new, so if someone has the correct details please feel free to correct me.
Then you will go on your RSW and hopefully smash it.
After that you will have to wait to get your Army number, which could take a while. Then you will get your kit.
So all in all, some aspects of the process take a very long time, as I'm sure you are aware.
You will still do regular TA weekends coming up to the Summer Challenge if you're booked onto it. For instance, I am required to do at least 1 weekend before I go on Summer Challenge. (However, I think the requirements are different for other Summer Challenges)
I should be on RSW this weekend, so after that I'll be waiting for my army number and kit, hoping to go onto my first weekend away before summer challenge. There is a chance that I won't make it on to Summer Challenge since the processes could take a while.
So it just depends how long things take really mate. Like I said, best thing to do would be to give them a call, or talk to someone when you take your RG8 forms in.
EDIT: Forgot to say what you do after summer challenge. Summer challenge is your phase 1 training, so if you complete that and pass out, you will then be on to phase 2. You can see here what you will have to expect for phase 2 training - http://www.arrse.co.uk/wiki/TA_Phase1_Training#Phase_2_Training
I'm not sure about the time frame, I think it's over a 2 week course, but again, feel free to correct me if I'm wrong.