It varied between cable companies- ours actually switched to the Sky version back in the autumn of 1999. Which was annoying as our cable company's version didn't charge to call them whereas the national version was premium rate. That and the addition of the DOG
were the indicators for us it had changed at the time.
The switch, on the national version at least, to the new system was September 2000. I'm sure some cable companies kept their own versions and the laserdisc system for a brief while after this. They probably didn't appreciate having to keep making laserdiscs for the cable company's systems. I wonder if any of those discs are still around, I presume a massive amount must have been made down the years.