How can I install a switch to power a bathroom TV off safely and discreetly?
You could go the route of having no switches in the bathroom, and only a plug and switch upstairs Loft.

Step 1: Get a standby plug adapter, which will shut off power to the TV when its in standby mode. this will reduce electrical risk, fire and save power. - the unit has an infra-red reset switch, so when it senses your remote IR beam, it will energise the TV up again. 

That way, all you need to do, is turn the TV off when you are done. and don't think about it anymore. 

Step 2:
You can actually look at the new Samsung Smart TV's with motion sensor on/off switches.  all you need to do is wave your hand near the Samsung logo and the TV Will switch on/off...