So, you have a set top box and want to watch it on more than one TV. |You don't care that it's on the same channel on all TVs at once and you don't want to pay for an additional box. If its a plug in pay as you rent streamer type gadget like a FireTV etc. & the cost is minimal, we would say get multiple of those, its easier, but if you're talking the big boxes from subscription based satellite or cable TV providers, there is a trick, fully legal and above board to let you stream that box throughout your house using your existing aerial network.
Sky used to have an RF & RF2 output on their old HD boxes (remember, you could attach this to your aerial booster box and tune in channel 6 or whatever analogue and voila, a fuzzy, low def broadcast of Sky on every TV with an aerial cable attached)
Well, now there are gadgets that do similar. Except.....instead of that fuzzy old analogue standard def picture with the wavy lines and free snowy ambience, you can broadcast the Sky in Full HD on a Freeview HD DIGITAL channel. Yes folks, full HD 1080p digital picture quality....not a snowflake or wavy line to be seen
What you need is an HDMI to DTV modulator. They're around £140- £180, so not cheap, but pay for themselves pretty quick. A few companies make these. Plug your box in with HDMI, loop back out with HDMI to wherever you had the HDMI in before, and take the RF feed to your aerial distribution amp (after plugging your aerial into the modulator of course)
Now it gets a bit tricky for the non-technicals out there. You need to set up a broadcast channel, name your channel, set the audio and video codecs (the defaults are normally fine) set the correct Freeview HD channel so you don't accidentally try to override actual TV channels in your area (if you still want these), then tune your TV, and voila , you will see your set-top box as a Freeview HD channel in the TV guide . Of course there are magic eyes so you can remote control the box from the TV rooms.
AV-Zone can help you with this if it sounds a bit daunting.