Rev matching is not required in modern vehicles for day to day driving, racing yes, going to Tesco no. It sounds like you have a worn synchromesh in 6th gear, very common problem and will cost £500+ to fix, so leave it until it becomes unbearable in more gears. The trick I found with going into 6th with a worn synchro in my old Zed was to put the car into neutral and release the clutch briefly, before putting it into 6th. 99% of the time it went in smoothly and only took an extra second.
As for having "no control of the car at all" if you're coasting in neutral, I guess these people don't have brakes or steering wheels. It's fine to do if thats what your dad wants, it won't do any harm to anyone or anything, it just uses more fuel and you have to apply brakes earlier.