I have this issue too on a UK model KDL32CX523BU. It started in the last week (so some time since 24th March) and doesn't affect the channels that we've had HD version of for ages (BBC1, BBC2, BBC Three, Channel 4) but only the new ones (BBC Four, BBC News, Al Jazeera English are the ones that spring to mind).
There really should be an option in the TV settings to turn this message off. I see a few threads (here and here) on the community dating back as far as 2012 asking for the feature but being fobbed off with the explanation that it's a part of the Freeview signal. That may be so, but it is totally within Sony's power as to what their TV software does with the signal. Another page here states that the prompt has been removed - well it plainly hasn't and as the starter of this thread notes it is very annoying.
If' the TV had behaved like this out of the box it would have been sent back by return for a refund. If some workaround can't be found I guess I'll be looking for a new TV.
