If I understand you correctly, there is no easy solution except to turn down the TV volume as you say, and route the sound elsewhere. Thing is, your TV is designed in such a way that you WANT to use its speakers for sound, at whatever TV volume you use, and presumably if you mute you get a constant on-screen icon reminding you that you killed the sound (this mute/volume setting wouldn't affect the audio out to your amp, but then you said you don't have that option). Guessing you bought a TV, not a monitor, but digiboxes output video and audio together for scart/aerial connections (s-video is picture only and needs seperate audio connections).
Of course, with a separates surround (and I face the same problem with sky/consoles/5.1 dvds) then you want to bypass the TV and feed straight into the amp, in which case the only way is to turn the TV volume down to defeat the TV speakers, and feed sound from the digibox etc straight to the amp )or via TV audio out). Digibox volume (check settings menus) and TV volume (as controlled by sky remote sending equivalent TV remote commands) are independent of each other.