You can disable One Power and still control the TV with the 360 remote.
The only difference with the setting disabled is you'll need to turn the TV on with the white on/off button on the remote and the 360 with the red (and turn them off the same way). The volume etc. will work no matter if One Power is on or off.
Also try setting the standby power to "fast start" and see if that improves things.
One of our 360's is connected to a Samsung TV (with One Power on!) and they both stay off when we turn them off (which is just as well as it's the bedroom TV!)
The settings we have on that particular 360 are as follows:
To get over the issue. I have to turn the virgin box off first then the TV. No random TV turn on. Is a solution. Had a few random times on turn on of virgin box it would not display. Had to reboot which still takes ages!
I have a Samsung tv which had a problem with turning on randomly. I have disabled the 'HDMI CEC' option which appears to stop the tv being turned on. Not sure how it works but it appears to be ok so far.
If you have more than one 360 you need the power setting to Fast or Active start so they can connect to each other so you can watch recordings across boxes and continue watching from one box to another.