Don't think a bigger bike won't blow about, I nearly ended up in the arnco on the M40 once on the Deauville...
If it was a 70, I'm guessing dual carriageway with a central divider, it's highly unlikely that it's going to be a 30, just one for the future. Other thing with speed limits - only 20, 40 and 50 need to be marked at regular intervals. 30, 60 and 70 don't. If you realise you're not in a 30, and there's no signs, then you're in a national speed limit, so pick up your speed, shows you've realised and rectified your mistake.
However - do make sure you at least do obvious mirror checks before opening the taps!