I've been driving for over 40 years and am doing less milage than ever now, but tyres just aren't lasting any where near as long as they used to.
This may be partly down to the amount of roundabouts in Milton Keynes where i live, but new tyres do not seem to have as much depth of tread or resiliance as they used to.
Legal limit always seems to be on the side of the manufacturer as i have driven in bad conditions with tyres well over their limit and have found them to still be safe.
In these days of mainly ABS braking systems i've found that ABS will tell you if a tyre is any good, on most road surfaces and weather conditions.
So i don't hold with scrapping a tyre before it has reached the wear bar on at least 1/3rd of the tyre.
Best advice given is to have vehicle tracking checked every time you see unusual wear, and check pressures regularly.
Remember that if your tyre wears unevenly nothing short of having them turned round on the rim will change their wear.
Phone round for best price, and tell them you'll decide whether you want other tyres or brakes done, its your money and car.
Try to pick an MOT station that is not going to profit from your vehicle failing, always helps to know your local guys unless you have loads of money to throw away.
The MOT guy will tell you if its over the limit or not whether you like it or not.
If your a long way from MOT time and your past the wear bar on half the tyre at least, dont wait for a policeman to tell you :)