In the dry you won't have problems provided you are driving within reasonable limits (i.e. you don't get the tyres screeching in braking/corners).
In the wet the problem you have is that the T1Rs will typically have better grip than the Advans (this changes if you manage to get tyre temperatures up a lot, but that's not likely in sane road driving in the wet).
So when you need to brake heavily (for example you misread the road ahead or there is something blocking the road) the Advans are going to lock up before the T1Rs.
As you have the advans on the front it's going to mean you get a load of understeer - not the end of the world, as long as you are competent to deal with it (and avoid it turning into oversteer as the grip returns; as the T1R has better grip in the wet, it's unlikely to be vicious oversteer).
If you had the T1Rs on the front, you would end up going into the hedge backwards. That's because a locked tyre has a fraction of the grip of a rolling one and hence doesn't provide the same braking effort to the car. You would be applying all your braking at the front and the back would be trying to overtake you - if you're anything other than perfectly straight, the car will do a 180 before you can do much about it.
Based on the above, you are right in saying that new T1Rs on the back + OK Advans on the front is a better and safer combination than bald Advans on the back + OK Advans on the front.
It's all common sense really
Enjoy the driving!
Robin
