This is not an uusual as when the toilet is flushed more cold water is drawn usually from the cold water pipe that suplies the shower with cold water so generaly the water under the shower gets hotter. The only way to stop this from happening is to lay an extra pipe all the way to the ater suply point, or fit a pump to maintain the presure
It won't change when you flush the toilet. :)
Each flush of a toilet uses the same amount of water.
No
It will be exaclty the same, the only difference is that flushing the toilet with bottled water is very expense. It really will be a waste of money.
When you take a shower, you're mixing a combination of hot and cold water to make a comfortable temperature for the shower water. A toilet draws only off the cold water system. In older or poorly designed plumbing systems, when you flush the toilet, the cold water is pulled by toilet, so the water coming through the shower is only drawing from the hot water tank.
There are temperature balanced shower bodies that will adjust the hot water flow when it senses that the cold water flow drops due to flushing the toilet.
Flushing a toilet
I reckon there is no air vent pipe on the drain line of the toilet you are flushing and the siphon action is sucking out the water in the loft toilet also.
to use less water when flushing
If the toilet and shower are in an upper floor then there is a PVC drain for both that will drain to either the sewer or septic. If there is a vertical piece to this, as there is in my basement, then you actually get some dribbling and running sounds from the water 'falling' down that vertical PVC. I get this same noise from the washing machine on the main floor draining down to the basement.
1.6 - 3.5 gpf
That is a matter of debate. Lets say you choose to have untreated water with harmful bacteria including the causes of typhoid and chlolera pumped into your toilet so you will not be wasteful. Then your toddler plays with the water in the toilet and gets sick and dies. I choose to be "wasteful".
Because it effects the water pressure of the shower. This causes the temperature of the water in the shower to also change.