Could somebody tell me if tapered threads on co2 extinguisher are still legal within the UK I know they was but getting different opinions 

Tapered threads are still used in Steel bodied extinguishers, which are still offered as economy models by a couple of manufacturers, but for decades now aluminium bodied extinguishers have used parallel threads - early aluminium body extinguishers in the late 60's and 70's did use tapered threads but were often beset with leaks & split necks.

There is actually very little law around extinguishers, most is just standards, however it's amazing how many non existent laws and standards are trotted out to sell new extinguishers.

If it's post 2002 and CE marked (unless refurbished) it will be legal.