At the Audi dealership I work at, service always try and give you a like for like or higher car than you currently have. Unfortunately it's not always as simple as that. The most recent example that sticks in my mind Is when a customer dropped their A1 in for a minor service, they had requested a loan car when they booked it in around two weeks before, which the service advisor they dealt with had forgotten, so they had to give mr customer their last loan car, which happened to be an RSQ3. The customer was obviously thrilled, but it wasn't ideal for the dealership because they wanted to keep the mileage down on it with it being an RS product.
Most of the time it's just luck of the draw. You could end up handing the keys to your £80,000 RS5 cabriolet over, in exchange for an A4 black edition multi!
My advice would be to specify a car you would like as a loan car at the point of booking, the dealership may not have one to give you, but if you give them notice they'll try their best to get you one.
It particularly helps if you mention that you're looking to change into the model you've requested!