Chris,

It is OK to use a tempering valve on a low pressure hot water system where the cold water is on mains pressure. The tempering valve or most shall I say, are fitted with a non-return valve to stop the cold water pushing through to the hot side.

As for your bathroom, you only need one tempering valve fitted to your system that supplies all your fixtures. This is generally fitted near to the hot water cylinder on the outlet side. You must ensure that tempered water is supplied to all your sanitary fixtures, not just your shower. Remember, the valve is factory set to 55 deg so the water is still hot enough for a bath or basin. Anything over 60 deg has the potential to scold.

Obviously, installing a tempering valve will temper all your fixtures throughout the house so in rare curcumstances, owners have a hot water feed prior to the tempering valve available to deliver to the kitchen or laundry tub, but not necessary.

Hope this helps

There are two types of tempering valves, one kind is only for mains pressure but you can also purchase a tempering valve for low pressure systems (larger mixing body and non-return).  These tempering valves are very common with retrofit solar systems on low pressure systems. Look at the APEX manufacturer specification.