The frame for the 340 does not support disc brakes and neither does the fork.
That being said there are products out there, not sold by Decathlon, that will convert your frame with a bolt on attatchment but you would still need to buy the brake. You would also need to replace the rear wheel for one that takes a rotor.
Parts plus labor your probably looking around the £150 mark so in my opinion not really worth it.
Hope this helps.