It certainly ended somewhere between 2001 and 2007. I remember where I was living when they stopped it.
When mail came twice a day, I used to get my post in the afternoon, consistently, and none in the morning, ever.
When they changed it, my post started turning up earlier, in the morning.
So it was weirdly an improvement. It seems strange now that there was ever two deliveries a day, given I get one letter a week at most now. (parcels are another matter, but again you don’t need Royal Mail delivering parcels twice a day even if you get a lot, so the fact they did this with letters is weird)